diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 276ee446..94e0856d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -30,7 +30,7 @@ jobs:
       - name: Install tools
         run: |
           python -m pip install --upgrade pip
-          pip install flake8 black isort
+          pip install flake8 black isort mypy
           pip install git+https://github.com/pyvisa/pyvisa.git#egg=pyvisa
       - name: Isort
         run: |
@@ -41,6 +41,9 @@ jobs:
       - name: Flake8
         run: |
           flake8 pyvisa-py;
+      - name: Mypy
+        run: |
+          mypy pyvisa;
   tests:
     name: Unit tests
     runs-on: ${{ matrix.os }}
@@ -86,5 +89,5 @@ jobs:
         if: needs.tests.result == 'success'
         run: exit 0
       - name: Mark the job as a failure
-        if: needs.tests.result == 'failure'
+        if: needs.tests.result != 'success'
         run: exit 1
diff --git a/pyvisa-py/common.py b/pyvisa-py/common.py
index 7242bdd9..3b9dff68 100644
--- a/pyvisa-py/common.py
+++ b/pyvisa-py/common.py
@@ -1,14 +1,12 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-sim.common
-    ~~~~~~~~~~~~~~~~~
+"""Common code.
 
-    Common code.
+:copyright: 2014-2020 by PyVISA-sim Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-sim Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import logging
+from typing import Optional, SupportsBytes
 
 from pyvisa import logger
 
@@ -16,25 +14,31 @@
 
 
 class MockInterface(object):
-    def __init__(self, resource_name):
+
+    #: Name of the resource used to create this interface
+    resource_name: str
+
+    def __init__(self, resource_name) -> None:
         self.resource_name = resource_name
 
 
 class NamedObject(object):
-    """A class to construct named sentinels.
-    """
+    """A class to construct named sentinels."""
 
-    def __init__(self, name):
+    #: Name used to identify the sentinel
+    name: str
+
+    def __init__(self, name) -> None:
         self.name = name
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "<%s>" % self.name
 
     __str__ = __repr__
 
 
-def iter_bytes(data, mask=None, send_end=False):
-
+# XXX can probably be removed
+def iter_bytes(data: SupportsBytes, mask: Optional[int] = None, send_end: bool = False):
     if send_end and mask is None:
         raise ValueError("send_end requires a valid mask.")
 
diff --git a/pyvisa-py/gpib.py b/pyvisa-py/gpib.py
index d2874167..3ef729e1 100644
--- a/pyvisa-py/gpib.py
+++ b/pyvisa-py/gpib.py
@@ -1,21 +1,21 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.gpib
-    ~~~~~~~~~~~~~~
+"""GPIB Session implementation using linux-gpib or gpib-ctypes.
 
-    GPIB Session implementation using linux-gpib or gpib-ctypes.
 
+:copyright: 2015-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2015-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import ctypes  # Used for missing bindings not ideal
 from bisect import bisect
+from typing import Any, Iterator, List, Optional, Tuple
 
 from pyvisa import attributes, constants, logger
+from pyvisa.constants import StatusCode, ResourceAttribute
 
 from .sessions import Session, UnknownAttribute
 
+
 try:
     GPIB_CTYPES = True
     from gpib_ctypes import gpib
@@ -52,7 +52,7 @@
 
 
 # patch Gpib to avoid double closing of handles
-def _patch_Gpib():
+def _patch_Gpib() -> None:
     if not hasattr(Gpib, "close"):
         _old_del = Gpib.__del__
 
@@ -67,9 +67,8 @@ def _inner(self):
 _patch_Gpib()
 
 
-def _find_boards():
-    """Find GPIB board addresses.
-    """
+def _find_boards() -> Iterator[Tuple[int, int]]:
+    """Find GPIB board addresses."""
     for board in range(16):
         try:
             yield board, gpib.ask(board, 1)
@@ -77,9 +76,8 @@ def _find_boards():
             logger.debug("GPIB board %i error in _find_boards(): %s", board, repr(e))
 
 
-def _find_listeners():
-    """Find GPIB listeners.
-    """
+def _find_listeners() -> Iterator[Tuple[int, int]]:
+    """Find GPIB listeners."""
     for board, boardpad in _find_boards():
         for i in range(31):
             try:
@@ -94,9 +92,24 @@ def _find_listeners():
                 )
 
 
-def _analyse_lines_value(value, line):
+def _analyse_lines_value(value: int, line: int):
     """Determine the state of a GPIB line based on the iblines byte.
 
+    Parameters
+    ----------
+    value : int
+        Value returned by iblines.
+    line : int
+        One of constants.VI_ATTR_GPIB_***_STATE where the *** can be REN, ATN,
+        NDAC, or SRQ
+
+    Returns
+    -------
+    constants.LineState
+        State of the line
+    StatusCode
+        Library StatusCode for the operation.
+
     """
     if line == constants.VI_ATTR_GPIB_REN_STATE:
         # REN bit valid = 0x10, REN bit value = 0x100
@@ -116,15 +129,13 @@ def _analyse_lines_value(value, line):
         value_mask = 0x2000
 
     if not value & validity_mask:
-        return constants.VI_STATE_UNKNOWN, StatusCode.success
+        return constants.LineState.unknown, StatusCode.success
     else:
         if value & value_mask:
-            return constants.VI_STATE_ASSERTED, StatusCode.success
+            return constants.LineState.asserted, StatusCode.success
         else:
-            return constants.VI_STATE_UNASSERTED, StatusCode.success
-
+            return constants.LineState.unasserted, StatusCode.success
 
-StatusCode = constants.StatusCode
 
 # linux-gpib timeout constants, in seconds. See GPIBSession._set_timeout.
 TIMETABLE = (
@@ -149,22 +160,29 @@ def _analyse_lines_value(value, line):
 )
 
 
-def convert_gpib_error(error, status, operation):
+def convert_gpib_error(
+    error: gpib.GpibError, status: int, operation: str
+) -> StatusCode:
     """Convert a GPIB error to a VISA StatusCode.
 
-    :param error: Error to use to determine the proper status code.
-    :type error: gpib.GpibError
-    :param status: Status byte of the GPIB library.
-    :type status: int
-    :param operation: Name of the operation that caused an exception. Used in logging.
-    :type operation: str
-    :return: Status code matching the GPIB error.
-    :rtype: constants.StatusCode
+    Parameters
+    ----------
+    error : gpib.GpibError
+        Error to use to determine the proper status code.
+    status : int
+        Status byte of the GPIB library.
+    operation : str
+        Name of the operation that caused an exception. Used in logging.
+
+    Returns
+    -------
+    StatusCode
+        Status code matching the GPIB error.
 
     """
     # First check the imeout condition in the status byte
     if status & 0x4000:
-        return constants.StatusCode.error_timeout
+        return StatusCode.error_timeout
     # All other cases are hard errors.
     # In particular linux-gpib simply gives a string we could parse but that
     # feels brittle. As a consequence we only try to be smart when using
@@ -172,33 +190,33 @@ def convert_gpib_error(error, status, operation):
     else:
         logger.debug("Failed to %s.", exc_info=error)
         if not GPIB_CTYPES:
-            return constants.StatusCode.error_system_error
+            return StatusCode.error_system_error
         if error.code == 1:
-            return constants.StatusCode.error_not_cic
+            return StatusCode.error_not_cic
         elif error.code == 2:
-            return constants.StatusCode.error_no_listeners
+            return StatusCode.error_no_listeners
         elif error.code == 4:
-            return constants.StatusCode.error_invalid_mode
+            return StatusCode.error_invalid_mode
         elif error.code == 11:
-            return constants.StatusCode.error_nonsupported_operation
+            return StatusCode.error_nonsupported_operation
         elif error.code == 1:
-            return constants.StatusCode.error_not_cic
+            return StatusCode.error_not_cic
         elif error.code == 21:
-            return constants.StatusCode.error_resource_locked
+            return StatusCode.error_resource_locked
         else:
-            return constants.StatusCode.error_system_error
+            return StatusCode.error_system_error
 
 
-def convert_gpib_status(status):
+def convert_gpib_status(status: int) -> StatusCode:
     if status & 0x4000:
-        return constants.StatusCode.error_timeout
+        return StatusCode.error_timeout
     elif status & 0x8000:
-        return constants.StatusCode.error_system_error
+        return StatusCode.error_system_error
     else:
-        return constants.StatusCode.success
+        return StatusCode.success
 
 
-class _GPIBCommon(object):
+class _GPIBCommon:
     """Common base class for GPIB sessions.
 
     Both INSTR and INTFC resources share the following attributes:
@@ -219,8 +237,11 @@ class _GPIBCommon(object):
 
     """
 
+    #: Bus wide controller.
+    controller: Gpib
+
     @classmethod
-    def get_low_level_info(cls):
+    def get_low_level_info(cls) -> str:
         try:
             ver = gpib.version()
         except AttributeError:
@@ -228,7 +249,7 @@ def get_low_level_info(cls):
 
         return "via Linux GPIB (%s)" % ver
 
-    def after_parsing(self):
+    def after_parsing(self) -> None:
         minor = int(self.parsed.board)
         sad = 0
         timeout = 13
@@ -259,7 +280,7 @@ def after_parsing(self):
             attribute = getattr(constants, "VI_ATTR_" + name)
             self.attrs[attribute] = attributes.AttributesByID[attribute].default
 
-    def _get_timeout(self, attribute):
+    def _get_timeout(self, attribute: constants.ResourceAttribute) -> Optional[int]:
         if self.interface:
             # 0x3 is the hexadecimal reference to the IbaTMO (timeout) configuration
             # option in linux-gpib.
@@ -271,8 +292,11 @@ def _get_timeout(self, attribute):
                 self.timeout = None
         return super(_GPIBCommon, self)._get_timeout(attribute)
 
-    def _set_timeout(self, attribute, value):
-        """
+    def _set_timeout(
+        self, attribute: constants.ResourceAttribute, value: Optional[int]
+    ):
+        """Set the timeout value.
+
         linux-gpib only supports 18 discrete timeout values. If a timeout
         value other than these is requested, it will be rounded up to the closest
         available value. Values greater than the largest available timout value
@@ -295,6 +319,7 @@ def _set_timeout(self, attribute, value):
         15  100 seconds
         16  300 seconds
         17  1000 seconds
+
         """
         status = super(_GPIBCommon, self)._set_timeout(attribute, value)
         if self.interface:
@@ -307,19 +332,28 @@ def _set_timeout(self, attribute, value):
             self.interface.timeout(gpib_timeout)
         return status
 
-    def close(self):
+    def close(self) -> None:
         if self.interface:
             self.interface.close()
         self.controller.close()
 
-    def read(self, count):
+    def read(self, count: int) -> bytes:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: bytes, constants.StatusCode
+        Parameters
+        ----------
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Data read from the interface.
+        StatusCode
+            Return value of the library call.
+
         """
         # INTFC don't have an interface so use the controller
         ifc = self.interface or self.controller
@@ -331,15 +365,23 @@ def read(self, count):
 
         return self._read(reader, count, checker, False, None, False, gpib.GpibError)
 
-    def write(self, data):
+    def write(self, data: bytes) -> Tuple[int, StatusCode]:
         """Writes data to device or interface synchronously.
 
         Corresponds to viWrite function of the VISA library.
 
-        :param data: data to be written.
-        :type data: bytes
-        :return: Number of bytes actually transferred, return value of the library call.
-        :rtype: int, VISAStatus
+        Parameters
+        ----------
+        data : bytes
+            Data to be written.
+
+        Returns
+        -------
+        int
+            Number of bytes actually transferred
+        StatusCode
+            Return value of the library call.
+
         """
         logger.debug("GPIB.write %r" % data)
 
@@ -354,17 +396,24 @@ def write(self, data):
         except gpib.GpibError as e:
             return 0, convert_gpib_error(e, ifc.ibsta(), "write")
 
-    def gpib_control_ren(self, mode):
-        """Controls the state of the GPIB Remote Enable (REN) interface line, and optionally the remote/local
-        state of the device.
+    def gpib_control_ren(self, mode: constants.RENLineOperation) -> StatusCode:
+        """Controls the state of the GPIB Remote Enable (REN) interface line.
+
+        Optionally the remote/local state of the device is also controlled.
 
         Corresponds to viGpibControlREN function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param mode: Specifies the state of the REN line and optionally the device remote/local state.
-                     (Constants.VI_GPIB_REN*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        mode : constants.RENLineOperation
+            Specifies the state of the REN line and optionally the device
+            remote/local state.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         if self.parsed.interface_type == "INTFC":
             if mode not in (
@@ -409,14 +458,23 @@ def gpib_control_ren(self, mode):
 
         return constants.StatusCode.success
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(self, attribute: ResourceAttribute) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource, return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Attribute for which the state query is made
+
+        Returns
+        -------
+        Any
+            The state of the queried attribute for a specified resource
+        StatusCode
+            Return value of the library call.
+
         """
         # TODO implement the following attributes
         # - VI_ATTR_INTF_INST_NAME RO
@@ -428,20 +486,20 @@ def _get_attribute(self, attribute):
         # INTFC don't have an interface so use the controller
         ifc = self.interface or self.controller
 
-        if attribute == constants.VI_ATTR_GPIB_PRIMARY_ADDR:
+        if attribute == ResourceAttribute.gpib_primary_address:
             # IbaPAD 0x1
             return ifc.ask(1), StatusCode.success
 
-        elif attribute == constants.VI_ATTR_GPIB_SECONDARY_ADDR:
+        elif attribute == ResourceAttribute.gpib_secondary_address:
             # IbaSAD 0x2
             # Remove 0x60 because National Instruments.
-            sad = ifc.ask(2)  # noqa
+            _ = ifc.ask(2)
             if ifc.ask(2):
                 return ifc.ask(2) - 96, StatusCode.success
             else:
                 return constants.VI_NO_SEC_ADDR, StatusCode.success
 
-        elif attribute == constants.VI_ATTR_GPIB_REN_STATE:
+        elif attribute == ResourceAttribute.gpib_ren_state:
             try:
                 lines = self.controller.lines()
                 return _analyse_lines_value(lines, attribute)
@@ -449,7 +507,7 @@ def _get_attribute(self, attribute):
                 # some versions of linux-gpib do not expose Gpib.lines()
                 return constants.VI_STATE_UNKNOWN, StatusCode.success
 
-        elif attribute == constants.VI_ATTR_SEND_END_EN:
+        elif attribute == ResourceAttribute.send_end_enabled:
             # Do not use IbaEndBitIsNormal 0x1a which relates to EOI on read()
             # not write(). see issue #196
             # IbcEOT 0x4
@@ -458,24 +516,34 @@ def _get_attribute(self, attribute):
             else:
                 return constants.VI_FALSE, StatusCode.success
 
-        elif attribute == constants.VI_ATTR_INTF_NUM:
+        elif attribute == ResourceAttribute.interface_number:
             # IbaBNA 0x200
             return ifc.ask(512), StatusCode.success
 
-        elif attribute == constants.VI_ATTR_INTF_TYPE:
+        elif attribute == ResourceAttribute.interface_type:
             return constants.InterfaceType.gpib, StatusCode.success
 
         raise UnknownAttribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: constants.ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Attribute for which the state is to be modified. (Attributes.*)
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         # TODO implement the following attributes
         # - VI_ATTR_DMA_ALLOW_EN RW
@@ -486,7 +554,7 @@ def _set_attribute(self, attribute, attribute_state):
         # INTFC don't have an interface so use the controller
         ifc = self.interface or self.controller
 
-        if attribute == constants.VI_ATTR_GPIB_READDR_EN:
+        if attribute == ResourceAttribute.gpib_readdress_enabled:
             # IbcREADDR 0x6
             # Setting has no effect in linux-gpib.
             if isinstance(attribute_state, int):
@@ -495,7 +563,7 @@ def _set_attribute(self, attribute, attribute_state):
             else:
                 return StatusCode.error_nonsupported_attribute_state
 
-        elif attribute == constants.VI_ATTR_GPIB_PRIMARY_ADDR:
+        elif attribute == ResourceAttribute.gpib_primary_address:
             # IbcPAD 0x1
             if isinstance(attribute_state, int) and 0 <= attribute_state <= 30:
                 ifc.config(1, attribute_state)
@@ -503,7 +571,7 @@ def _set_attribute(self, attribute, attribute_state):
             else:
                 return StatusCode.error_nonsupported_attribute_state
 
-        elif attribute == constants.VI_ATTR_GPIB_SECONDARY_ADDR:
+        elif attribute == ResourceAttribute.gpib_secondary_address:
             # IbcSAD 0x2
             # Add 0x60 because National Instruments.
             if isinstance(attribute_state, int) and 0 <= attribute_state <= 30:
@@ -515,7 +583,7 @@ def _set_attribute(self, attribute, attribute_state):
             else:
                 return StatusCode.error_nonsupported_attribute_state
 
-        elif attribute == constants.VI_ATTR_GPIB_UNADDR_EN:
+        elif attribute == ResourceAttribute.gpib_unadress_enable:
             # IbcUnAddr 0x1b
             try:
                 ifc.config(27, attribute_state)
@@ -523,7 +591,7 @@ def _set_attribute(self, attribute, attribute_state):
             except gpib.GpibError:
                 return StatusCode.error_nonsupported_attribute_state
 
-        elif attribute == constants.VI_ATTR_SEND_END_EN:
+        elif attribute == ResourceAttribute.send_end_enabled:
             # Do not use IbaEndBitIsNormal 0x1a which relates to EOI on read()
             # not write(). see issue #196
             # IbcEOT 0x4
@@ -539,23 +607,23 @@ def _set_attribute(self, attribute, attribute_state):
 # TODO: Check secondary addresses.
 @Session.register(constants.InterfaceType.gpib, "INSTR")
 class GPIBSession(_GPIBCommon, Session):
-    """A GPIB Session that uses linux-gpib to do the low level communication.
-    """
+    """A GPIB Session that uses linux-gpib to do the low level communication."""
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         return ["GPIB%d::%d::INSTR" % (board, pad) for board, pad in _find_listeners()]
 
-    def clear(self):
+    def clear(self) -> StatusCode:
         """Clears a device.
 
         Corresponds to viClear function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
-        """
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
 
+        """
         logger.debug("GPIB.device clear")
         try:
             self.interface.clear()
@@ -563,14 +631,21 @@ def clear(self):
         except gpib.GpibError as e:
             return convert_gpib_error(e, self.interface.ibsta(), "clear")
 
-    def assert_trigger(self, protocol):
+    def assert_trigger(self, protocol: constants.TriggerProtocol) -> StatusCode:
         """Asserts hardware trigger.
-        Only supports protocol = constants.VI_TRIG_PROT_DEFAULT
 
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
-        """
+        Parameters
+        ----------
+        protocol : constants.TriggerProtocol
+            Triggering protocol to use.
+            Only supports constants.TriggerProtocol.default
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
 
+        """
         logger.debug("GPIB.device assert hardware trigger")
 
         try:
@@ -582,13 +657,16 @@ def assert_trigger(self, protocol):
         except gpib.GpibError as e:
             return convert_gpib_error(e, self.interface.ibsta(), "assert trigger")
 
-    def read_stb(self):
+    def read_stb(self) -> Tuple[int, StatusCode]:
+        """Read the device status byte."""
         try:
             return self.interface.serial_poll(), StatusCode.success
         except gpib.GpibError as e:
             return 0, convert_gpib_error(e, self.interface.ibsta(), "read STB")
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(
+        self, attribute: constants.ResourceAttribute
+    ) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes. GPIB::INSTR have the
@@ -600,9 +678,18 @@ def _get_attribute(self, attribute):
         - VI_ATTR_GPIB_READDR_EN
         - VI_ATTR_GPIB_UNADDR_EN
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource, return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Resource attribute for which the state query is made
+
+        Returns
+        -------
+        Any
+            State of the queried attribute for a specified resource
+        StatusCode
+            Return value of the library call.
+
         """
         # TODO implement the following attributes
         # - VI_ATTR_TRIG_ID RW or RO see specs
@@ -624,15 +711,25 @@ def _get_attribute(self, attribute):
 
         return super(GPIBSession, self)._get_attribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Attribute for which the state is to be modified. (Attributes.*)
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         # TODO implement the following attributes
         # - VI_ATTR_TRIG_ID RW or RO see specs
@@ -666,33 +763,38 @@ class GPIBInterface(_GPIBCommon, Session):
     """
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         return ["GPIB%d::INTFC" % board for board, pad in _find_boards()]
 
-    def gpib_command(self, command_bytes):
+    def gpib_command(self, command_bytes: bytes) -> Tuple[int, StatusCode]:
         """Write GPIB command byte on the bus.
 
         Corresponds to viGpibCommand function of the VISA library.
         See: https://linux-gpib.sourceforge.io/doc_html/gpib-protocol.html#REFERENCE-COMMAND-BYTES
 
-        :param command_bytes: command bytes to send
-        :type command_bytes: bytes
-        :return: Number of written bytes, return value of the library call.
-        :rtype: int, :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        command_bytes : bytes
+            Command bytes to send
+
+        Returns
+        -------
+        int
+            Number of written bytes,
+        StatusCode
+            Return value of the library call.
+
         """
         try:
             return self.controller.command(command_bytes), StatusCode.success
         except gpib.GpibError as e:
             return convert_gpib_error(e, self.controller.ibsta(), "gpib command")
 
-    def gpib_send_ifc(self):
+    def gpib_send_ifc(self) -> StatusCode:
         """Pulse the interface clear line (IFC) for at least 100 microseconds.
 
         Corresponds to viGpibSendIFC function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
         """
         logger.debug("GPIB.interface clear")
         try:
@@ -701,16 +803,22 @@ def gpib_send_ifc(self):
         except gpib.GpibError as e:
             return convert_gpib_error(e, self.controller.ibsta(), "send IFC")
 
-    def gpib_control_atn(self, mode):
+    def gpib_control_atn(self, mode: constants.ATNLineOperation) -> StatusCode:
         """Specifies the state of the ATN line and the local active controller state.
 
         Corresponds to viGpibControlATN function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param mode: Specifies the state of the ATN line and optionally the local active controller state.
-                     (Constants.VI_GPIB_ATN*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        mode : constants.ATNLineOperation
+            Specifies the state of the ATN line and optionally the local active
+             controller state.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         logger.debug("GPIB.control atn")
         if mode == constants.VI_GPIB_ATN_ASSERT:
@@ -726,18 +834,27 @@ def gpib_control_atn(self, mode):
             return constants.StatusCode.error_invalid_mode
         return convert_gpib_status(status)
 
-    def gpib_pass_control(self, primary_address, secondary_address):
-        """Tell the GPIB device at the specified address to become controller in charge (CIC).
+    def gpib_pass_control(
+        self, primary_address: int, secondary_address: int
+    ) -> StatusCode:
+        """Tell a GPIB device to become controller in charge (CIC).
 
         Corresponds to viGpibPassControl function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param primary_address: Primary address of the GPIB device to which you want to pass control.
-        :param secondary_address: Secondary address of the targeted GPIB device.
-                                  If the targeted device does not have a secondary address,
-                                  this parameter should contain the value Constants.VI_NO_SEC_ADDR.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        primary_address : int
+            Primary address of the GPIB device to which you want to pass control.
+        secondary_address : int
+            Secondary address of the targeted GPIB device.
+            If the targeted device does not have a secondary address,
+            this parameter should contain the value Constants.VI_NO_SEC_ADDR.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         # ibpct need to get the device id matching the primary and secondary address
         logger.debug("GPIB.pass control")
@@ -752,7 +869,7 @@ def gpib_pass_control(self, primary_address, secondary_address):
         status = gpib_lib.ibpct(did)
         return convert_gpib_status(status)
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(self, attribute: ResourceAttribute) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes. GPIB::INTFC have the
@@ -767,9 +884,18 @@ def _get_attribute(self, attribute):
         - VI_ATTR_GPIB_HS488_CBL_LEN
         - VI_ATTR_GPIB_ADDR_STATE
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource, return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
+        Parameters
+        ----------
+        attribute: ResourceAttribute
+            Resource attribute for which the state query is made.
+
+        Returns
+        -------
+        Any
+            The state of the queried attribute for a specified resource,
+        StatusCode
+            Return value of the library call.
+
         """
         # TODO implement the following attributes
         # - VI_ATTR_DEV_STATUS_BYTE RW
@@ -799,21 +925,31 @@ def _get_attribute(self, attribute):
 
         return super(GPIBSession, self)._get_attribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Attribute for which the state is to be modified.
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         # TODO implement the following attributes
         # - VI_ATTR_GPIB_SYS_CNTRL_STATE
         # - VI_ATTR_DEV_STATUS_BYTE
 
         # INTFC don't have an interface so use the controller
-        ifc = self.controller  # noqa
+        _ = self.controller
 
         return super(GPIBSession, self)._set_attribute(attribute, attribute_state)
diff --git a/pyvisa-py/highlevel.py b/pyvisa-py/highlevel.py
index 3282487a..cf9bbc56 100644
--- a/pyvisa-py/highlevel.py
+++ b/pyvisa-py/highlevel.py
@@ -1,38 +1,39 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.highlevel
-    ~~~~~~~~~~~~~~~~~~~
+"""Highlevel wrapper of the VISA Library.
 
-    Highlevel wrapper of the VISA Library.
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import random
 from collections import OrderedDict
+from typing import Any, Dict, Iterable, Iterator, Optional, Tuple, Union
 
 from pyvisa import constants, errors, highlevel, rname
+from pyvisa.constants import StatusCode
+from pyvisa.typing import VISAEventContext, VISARMSession, VISASession
 
 from . import sessions
 from .common import logger
 
-StatusCode = constants.StatusCode
-
 
 class PyVisaLibrary(highlevel.VisaLibraryBase):
     """A pure Python backend for PyVISA.
 
     The object is basically a dispatcher with some common functions implemented.
 
-    When a new resource object is requested to pyvisa, the library creates a Session object
-    (that knows how to perform low-level communication operations) associated with a session handle
-    (a number, usually refered just as session).
+    When a new resource object is requested to pyvisa, the library creates a
+    Session object (that knows how to perform low-level communication operations)
+    associated with a session handle (a number, usually refered just as session).
 
-    A call to a library function is handled by PyVisaLibrary if it involves a resource agnosting
-    function or dispatched to the correct session object (obtained from the session id).
+    A call to a library function is handled by PyVisaLibrary if it involves a
+    resource agnostic function or dispatched to the correct session object
+    (obtained from the session id).
+
+    Importantly, the user is unaware of this. PyVisaLibrary behaves for
+    the user just as NIVisaLibrary.
 
-    Importantly, the user is unaware of this. PyVisaLibrary behaves for the user just as NIVisaLibrary.
     """
 
     # Try to import packages implementing lower level functionality.
@@ -65,22 +66,23 @@ class PyVisaLibrary(highlevel.VisaLibraryBase):
         logger.debug("GPIBSession was not imported %s." % e)
 
     @classmethod
-    def get_session_classes(cls):
+    def get_session_classes(cls) -> Dict[sessions.Session]:
         return sessions.Session._session_classes
 
     @classmethod
-    def iter_session_classes_issues(cls):
+    def iter_session_classes_issues(
+        cls,
+    ) -> Iterator[Tuple[constants.InterfaceType, str], str]:
         return sessions.Session.iter_session_classes_issues()
 
     @staticmethod
-    def get_library_paths():
+    def get_library_paths() -> Iterable[str]:
         """List a dummy library path to allow to create the library."""
         return ("py",)
 
     @staticmethod
-    def get_debug_info():
-        """Return a list of lines with backend info.
-        """
+    def get_debug_info() -> Dict[str, Union[str, Dict[str, str]]]:
+        """Return a list of lines with backend info."""
         from . import __version__
 
         d = OrderedDict()
@@ -95,19 +97,17 @@ def get_debug_info():
 
         return d
 
-    def _init(self):
-
+    def _init(self) -> None:
+        """Custom initialization code."""
         #: map session handle to session object.
         #: dict[int, session.Session]
         self.sessions = {}
 
-    def _register(self, obj):
-        """Creates a random but unique session handle for a session object,
-        register it in the sessions dictionary and return the value
+    def _register(self, obj: object) -> VISASession:
+        """Creates a random but unique session handle for a session object.
+
+        Register it in the sessions dictionary and return the value.
 
-        :param obj: a session object.
-        :return: session handle
-        :rtype: int
         """
         session = None
 
@@ -120,24 +120,36 @@ def _register(self, obj):
     # noinspection PyShadowingBuiltins
     def open(
         self,
-        session,
-        resource_name,
-        access_mode=constants.AccessModes.no_lock,
-        open_timeout=constants.VI_TMO_IMMEDIATE,
-    ):
+        session: VISARMSession,
+        resource_name: str,
+        access_mode: constants.AccessModes = constants.AccessModes.no_lock,
+        open_timeout: Optional[int] = constants.VI_TMO_IMMEDIATE,
+    ) -> Tuple[VISASession, StatusCode]:
         """Opens a session to the specified resource.
 
         Corresponds to viOpen function of the VISA library.
 
-        :param session: Resource Manager session (should always be a session returned from open_default_resource_manager()).
-        :param resource_name: Unique symbolic name of a resource.
-        :param access_mode: Specifies the mode by which the resource is to be accessed. (constants.AccessModes)
-        :param open_timeout: Specifies the maximum time period (in milliseconds) that this operation waits
-                             before returning an error.
-        :return: Unique logical identifier reference to a session, return value of the library call.
-        :rtype: session, VISAStatus
-        """
+        Parameters
+        ----------
+        session : typing.VISARMSession
+            Resource Manager session (should always be a session returned from
+            open_default_resource_manager()).
+        resource_name : str
+            Unique symbolic name of a resource.
+        access_mode : constants.AccessModes, optional
+            Specifies the mode by which the resource is to be accessed.
+        open_timeout : Optional[int]
+            Specifies the maximum time period (in milliseconds) that this
+            operation waits before returning an error.
+
+        Returns
+        -------
+        typing.VISASession
+            Unique logical identifier reference to a session
+        StatusCode
+            Return value of the library call.
 
+        """
         try:
             open_timeout = int(open_timeout)
         except ValueError:
@@ -159,14 +171,21 @@ def open(
 
         return self._register(sess), StatusCode.success
 
-    def clear(self, session):
+    def clear(self, session: VISASession) -> StatusCode:
         """Clears a device.
 
         Corresponds to viClear function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : typin.VISASession
+            Unique logical identifier to a session.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -174,17 +193,30 @@ def clear(self, session):
             return constants.StatusCode.error_invalid_object
         return sess.clear()
 
-    def flush(self, session, mask):
-        """Flushes device buffers.
+    def flush(
+        self, session: VISASession, mask: constants.BufferOperation
+    ) -> constants.StatusCode:
+        """Flush the specified buffers.
+
+        The buffers can be associated with formatted I/O operations and/or
+        serial communication.
+
+        Corresponds to viFlush function of the VISA library.
 
-        Corresponds to viFlush function of the VISA library. See:
-        https://pyvisa.readthedocs.io/en/latest/api/visalibrarybase.html?highlight=flush#pyvisa.highlevel.VisaLibraryBase.flush
-        for valid values of mask.
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        mask : constants.BufferOperation
+            Specifies the action to be taken with flushing the buffer.
+            The values can be combined using the | operator. However multiple
+            operations on a single buffer cannot be combined.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
 
-        :param session: Unique logical identifier to a session.
-        :param mask: which buffers to clear.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
         """
         try:
             sess = self.sessions[session]
@@ -192,45 +224,73 @@ def flush(self, session, mask):
             return constants.StatusCode.error_invalid_object
         return sess.flush(mask)
 
-    def gpib_command(self, session, command_byte):
-        """Write GPIB command byte on the bus.
+    def gpib_command(
+        self, session: VISASession, command_byte: bytes
+    ) -> Tuple[int, constants.StatusCode]:
+        """Write GPIB command bytes on the bus.
 
         Corresponds to viGpibCommand function of the VISA library.
-        See: https://linux-gpib.sourceforge.io/doc_html/gpib-protocol.html#REFERENCE-COMMAND-BYTES
 
-        :param command_byte: command byte to send
-        :type command_byte: int, must be [0 255]
-        :return: return value of the library call
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        command_byte : bytes
+            Data to write.
+
+        Returns
+        -------
+        int
+            Number of written bytes
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             return self.sessions[session].gpib_command(command_byte)
         except KeyError:
             return constants.StatusCode.error_invalid_object
 
-    def assert_trigger(self, session, protocol):
-        """Asserts software or hardware trigger.
+    def assert_trigger(
+        self, session: VISASession, protocol: constants.TriggerProtocol
+    ) -> constants.StatusCode:
+        """Assert software or hardware trigger.
 
         Corresponds to viAssertTrigger function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param protocol: Trigger protocol to use during assertion. (Constants.PROT*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        protocol : constants.TriggerProtocol
+            Trigger protocol to use during assertion.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             return self.sessions[session].assert_trigger(protocol)
         except KeyError:
             return constants.StatusCode.error_invalid_object
 
-    def gpib_send_ifc(self, session):
+    def gpib_send_ifc(self, session: VISASession) -> constants.StatusCode:
         """Pulse the interface clear line (IFC) for at least 100 microseconds.
 
         Corresponds to viGpibSendIFC function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -238,17 +298,27 @@ def gpib_send_ifc(self, session):
             return constants.StatusCode.error_invalid_object
         return sess.gpib_send_ifc()
 
-    def gpib_control_ren(self, session, mode):
-        """Controls the state of the GPIB Remote Enable (REN) interface line, and optionally the remote/local
-        state of the device.
+    def gpib_control_ren(
+        self, session: VISASession, mode: constants.RENLineOperation
+    ) -> constants.StatusCode:
+        """Controls the state of the GPIB Remote Enable (REN) interface line.
+
+        Optionally the remote/local state of the device can also be set.
 
         Corresponds to viGpibControlREN function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param mode: Specifies the state of the REN line and optionally the device remote/local state.
-                     (Constants.VI_GPIB_REN*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        mode : constants.RENLineOperation
+            State of the REN line and optionally the device remote/local state.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -256,16 +326,25 @@ def gpib_control_ren(self, session, mode):
             return constants.StatusCode.error_invalid_object
         return sess.gpib_control_ren()
 
-    def gpib_control_atn(self, session, mode):
+    def gpib_control_atn(
+        self, session: VISASession, mode: constants.ATNLineOperation
+    ) -> constants.StatusCode:
         """Specifies the state of the ATN line and the local active controller state.
 
         Corresponds to viGpibControlATN function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param mode: Specifies the state of the ATN line and optionally the local active controller state.
-                     (Constants.VI_GPIB_ATN*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        mode : constants.ATNLineOperation
+            State of the ATN line and optionally the local active controller state.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -273,18 +352,29 @@ def gpib_control_atn(self, session, mode):
             return constants.StatusCode.error_invalid_object
         return sess.gpib_control_atn()
 
-    def gpib_pass_control(self, session, primary_address, secondary_address):
-        """Tell the GPIB device at the specified address to become controller in charge (CIC).
+    def gpib_pass_control(
+        self, session: VISASession, primary_address: int, secondary_address: int
+    ) -> constants.StatusCode:
+        """Tell a GPIB device to become controller in charge (CIC).
 
         Corresponds to viGpibPassControl function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param primary_address: Primary address of the GPIB device to which you want to pass control.
-        :param secondary_address: Secondary address of the targeted GPIB device.
-                                  If the targeted device does not have a secondary address,
-                                  this parameter should contain the value Constants.VI_NO_SEC_ADDR.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        primary_address : int
+            Primary address of the GPIB device to which you want to pass control.
+        secondary_address : int
+            Secondary address of the targeted GPIB device.
+            If the targeted device does not have a secondary address, this parameter
+            should contain the value Constants.VI_NO_SEC_ADDR.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -292,12 +382,23 @@ def gpib_pass_control(self, session, primary_address, secondary_address):
             return constants.StatusCode.error_invalid_object
         return sess.gpib_pass_control()
 
-    def read_stb(self, session):
+    def read_stb(self, session: VISASession) -> Tuple[int, constants.StatusCode]:
         """Reads a status byte of the service request.
+
         Corresponds to viReadSTB function of the VISA library.
-        :param session: Unique logical identifier to a session.
-        :return: Service request status byte, return value of the library call.
-        :rtype: int, :class:`pyvisa.constants.StatusCode`
+
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+
+        Returns
+        -------
+        int
+            Service request status byte
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -305,14 +406,23 @@ def read_stb(self, session):
             return 0, constants.StatusCode.error_invalid_object
         return sess.read_stb()
 
-    def close(self, session):
+    def close(
+        self, session: Union[VISASession, VISAEventContext, VISARMSession]
+    ) -> constants.StatusCode:
         """Closes the specified session, event, or find list.
 
         Corresponds to viClose function of the VISA library.
 
-        :param session: Unique logical identifier to a session, event, or find list.
-        :return: return value of the library call.
-        :rtype: VISAStatus
+        Parameters
+        ---------
+        session : Union[VISASession, VISAEventContext, VISARMSession]
+            Unique logical identifier to a session, event, resource manager.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -321,22 +431,41 @@ def close(self, session):
         except KeyError:
             return StatusCode.error_invalid_object
 
-    def open_default_resource_manager(self):
+    def open_default_resource_manager(
+        self,
+    ) -> Tuple[VISARMSession, constants.StatusCode]:
         """This function returns a session to the Default Resource Manager resource.
 
         Corresponds to viOpenDefaultRM function of the VISA library.
 
-        :return: Unique logical identifier to a Default Resource Manager session, return value of the library call.
-        :rtype: session, VISAStatus
+        Returns
+        -------
+        VISARMSession
+            Unique logical identifier to a Default Resource Manager session
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return self._register(self), StatusCode.success
 
-    def list_resources(self, session, query="?*::INSTR"):
-        """Returns a tuple of all connected devices matching query.
+    def list_resources(
+        self, session: VISARMSession, query: str = "?*::INSTR"
+    ) -> Tuple[str, ...]:
+        """Return a tuple of all connected devices matching query.
 
-        :param query: regular expression used to match devices.
-        """
+        Parameters
+        ----------
+        session : VISARMSession
+            Unique logical identifier to the resource manager session.
+        query : str
+            Regular expression used to match devices.
 
+        Returns
+        -------
+        Tuple[str, ...]
+            Resource names of all the connected devices matching the query.
+
+        """
         # For each session type, ask for the list of connected resources and
         # merge them into a single list.
 
@@ -352,15 +481,27 @@ def list_resources(self, session, query="?*::INSTR"):
 
         return resources
 
-    def read(self, session, count):
+    def read(
+        self, session: VISASession, count: int
+    ) -> Tuple[bytes, constants.StatusCode]:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: bytes, VISAStatus
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Date read
+        constants.StatusCode
+            Return value of the library call.
+
         """
         # from the session handle, dispatch to the read method of the session object.
         try:
@@ -373,16 +514,27 @@ def read(self, session, count):
 
         return ret
 
-    def write(self, session, data):
-        """Writes data to device or interface synchronously.
+    def write(
+        self, session: VISASession, data: bytes
+    ) -> Tuple[int, constants.StatusCode]:
+        """Write data to device or interface synchronously.
 
         Corresponds to viWrite function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param data: data to be written.
-        :type data: str
-        :return: Number of bytes actually transferred, return value of the library call.
-        :rtype: int, VISAStatus
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        data : bytes
+            Data to be written.
+
+        Returns
+        -------
+        int
+            Number of bytes actually transferred
+        constants.StatusCode
+            Return value of the library call.
+
         """
         # from the session handle, dispatch to the write method of the session object.
         try:
@@ -395,38 +547,79 @@ def write(self, session, data):
 
         return ret
 
-    def buffer_read(self, session, count):
-        """Reads data from device or interface through the use of a formatted I/O read buffer.
+    def buffer_read(
+        self, session: VISASession, count: int
+    ) -> Tuple[bytes, constants.StatusCode]:
+        """Reads data through the use of a formatted I/O read buffer.
+
+        The data can be read from a device or an interface.
+
         Corresponds to viBufRead function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: bytes, VISAStatus
+        Parameters
+        ----------
+        session : VISASession\
+            Unique logical identifier to a session.
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Data read
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return self.read(session, count)
 
-    def buffer_write(self, session, data):
+    def buffer_write(
+        self, session: VISASession, data: bytes
+    ) -> Tuple[int, constants.StatusCode]:
         """Writes data to a formatted I/O write buffer synchronously.
+
         Corresponds to viBufWrite function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param data: data to be written.
-        :type data: str
-        :return: Number of bytes actually transferred, return value of the library call.
-        :rtype: int, VISAStatus
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        data : bytes
+            Data to be written.
+
+        Returns
+        -------
+        int
+            number of written bytes
+        constants.StatusCode
+            return value of the library call.
+
         """
         return self.write(session, data)
 
-    def get_attribute(self, session, attribute):
+    def get_attribute(
+        self,
+        session: Union[VISASession, VISAEventContext, VISARMSession],
+        attribute: Union[constants.ResourceAttribute, constants.EventAttribute],
+    ) -> Tuple[Any, constants.StatusCode]:
         """Retrieves the state of an attribute.
 
         Corresponds to viGetAttribute function of the VISA library.
 
-        :param session: Unique logical identifier to a session, event, or find list.
-        :param attribute: Resource attribute for which the state query is made (see Attributes.*)
-        :return: The state of the queried attribute for a specified resource, return value of the library call.
-        :rtype: unicode | str | list | int, VISAStatus
+        Parameters
+        ----------
+        session : Union[VISASession, VISAEventContext]
+            Unique logical identifier to a session, event, or find list.
+        attribute : Union[constants.ResourceAttribute, constants.EventAttribute]
+            Resource or event attribute for which the state query is made.
+
+        Returns
+        -------
+        Any
+            State of the queried attribute for a specified resource
+        constants.StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -435,18 +628,31 @@ def get_attribute(self, session, attribute):
 
         return sess.get_attribute(attribute)
 
-    def set_attribute(self, session, attribute, attribute_state):
-        """Sets the state of an attribute.
+    def set_attribute(
+        self,
+        session: VISASession,
+        attribute: constants.ResourceAttribute,
+        attribute_state: Any,
+    ) -> constants.StatusCode:
+        """Set the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
-        """
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        attribute : constants.ResourceAttribute
+            Attribute for which the state is to be modified.
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
 
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
+        """
         try:
             sess = self.sessions[session]
         except KeyError:
@@ -454,18 +660,38 @@ def set_attribute(self, session, attribute, attribute_state):
 
         return sess.set_attribute(attribute, attribute_state)
 
-    def lock(self, session, lock_type, timeout, requested_key=None):
+    def lock(
+        self,
+        session: VISASession,
+        lock_type: constants.Lock,
+        timeout: int,
+        requested_key: Optional[str] = None,
+    ) -> Tuple[str, constants.StatusCode]:
         """Establishes an access mode to the specified resources.
 
         Corresponds to viLock function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param lock_type: Specifies the type of lock requested, either Constants.EXCLUSIVE_LOCK or Constants.SHARED_LOCK.
-        :param timeout: Absolute time period (in milliseconds) that a resource waits to get unlocked by the
-                        locking session before returning an error.
-        :param requested_key: This parameter is not used and should be set to VI_NULL when lockType is VI_EXCLUSIVE_LOCK.
-        :return: access_key that can then be passed to other sessions to share the lock, return value of the library call.
-        :rtype: str, :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        lock_type : constants.Lock
+            Specifies the type of lock requested.
+        timeout : int
+            Absolute time period (in milliseconds) that a resource waits to get
+            unlocked by the locking session before returning an error.
+        requested_key : Optional[str], optional
+            Requested locking key in the case of a shared lock. For an exclusive
+            lock it should be None.
+
+        Returns
+        -------
+        Optional[str]
+            Key that can then be passed to other sessions to share the lock, or
+            None for an exclusive lock.
+        StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -474,14 +700,21 @@ def lock(self, session, lock_type, timeout, requested_key=None):
 
         return sess.lock(lock_type, timeout, requested_key)
 
-    def unlock(self, session):
-        """Relinquishes a lock for the specified resource.
+    def unlock(self, session: VISASession) -> constants.StatusCode:
+        """Relinquish a lock for the specified resource.
 
         Corresponds to viUnlock function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         try:
             sess = self.sessions[session]
@@ -490,10 +723,56 @@ def unlock(self, session):
 
         return sess.unlock()
 
-    def disable_event(self, session, event_type, mechanism):
-        # TODO: implement this for GPIB finalization
+    def disable_event(
+        self,
+        session: VISASession,
+        event_type: constants.EventType,
+        mechanism: constants.EventMechanism,
+    ) -> StatusCode:
+        """Disable notification for an event type(s) via the specified mechanism(s).
+
+        Corresponds to viDisableEvent function of the VISA library.
+
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        event_type : constants.EventType
+            Event type.
+        mechanism : constants.EventMechanism
+            Event handling mechanisms to be disabled.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
+        """
         pass
 
-    def discard_events(self, session, event_type, mechanism):
-        # TODO: implement this for GPIB finalization
+    def discard_events(
+        self,
+        session: VISASession,
+        event_type: constants.EventType,
+        mechanism: constants.EventMechanism,
+    ) -> StatusCode:
+        """Discard event occurrences for a given type and mechanisms in a session.
+
+        Corresponds to viDiscardEvents function of the VISA library.
+
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        event_type : constans.EventType
+            Logical event identifier.
+        mechanism : constants.EventMechanism
+            Specifies event handling mechanisms to be discarded.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
+        """
         pass
diff --git a/pyvisa-py/protocols/__init__.py b/pyvisa-py/protocols/__init__.py
index 1f3bf64c..2e843c03 100644
--- a/pyvisa-py/protocols/__init__.py
+++ b/pyvisa-py/protocols/__init__.py
@@ -1,11 +1,8 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.protocols
-    ~~~~~~~~~~~~~~~~~~~
+"""Implements protocols on top of lower level libraries to talk to instruments.
 
-    Implements protocols on top of lower level libraries to talk to instruments.
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
diff --git a/pyvisa-py/protocols/rpc.py b/pyvisa-py/protocols/rpc.py
index 9845e2b5..07349357 100644
--- a/pyvisa-py/protocols/rpc.py
+++ b/pyvisa-py/protocols/rpc.py
@@ -1,26 +1,22 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.protocols.rpc
-    ~~~~~~~~~~~~~~~~~~~~~~~
+"""Sun RPC version 2 -- RFC1057
 
-    Sun RPC version 2 -- RFC1057
+This file is drawn from Python's RPC demo, updated for python 3.
 
-    This file is drawn from Python's RPC demo, updated for python 3.
+XXX There should be separate exceptions for the various reasons why
+XXX an RPC can fail, rather than using RuntimeError for everything
 
-    XXX There should be separate exceptions for the various reasons why
-    XXX an RPC can fail, rather than using RuntimeError for everything
+XXX The UDP version of the protocol resends requests when it does
+XXX not receive a timely reply -- use only for idempotent calls!
 
-    XXX The UDP version of the protocol resends requests when it does
-    XXX not receive a timely reply -- use only for idempotent calls!
+Original source:
+    http://svn.python.org/projects/python/trunk/Demo/rpc/rpc.py
 
-    Original source:
-        http://svn.python.org/projects/python/trunk/Demo/rpc/rpc.py
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
-
 import enum
 import select
 import socket
diff --git a/pyvisa-py/protocols/usbraw.py b/pyvisa-py/protocols/usbraw.py
index 970da9b3..04c7c96a 100644
--- a/pyvisa-py/protocols/usbraw.py
+++ b/pyvisa-py/protocols/usbraw.py
@@ -1,17 +1,14 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.protocols.usbraw
-    ~~~~~~~~~~~~~~~~~~~~~~~~~~
+"""Implements Session to control USB Raw devices
 
-    Implements Session to control USB Raw devices
+Loosely based on PyUSBTMC:python module to handle
+USB-TMC(Test and Measurement class) devices. by Noboru Yamamot, Accl. Lab, KEK, JAPAN
 
-    Loosely based on PyUSBTMC:python module to handle USB-TMC(Test and Measurement class)　devices.
-    by Noboru Yamamot, Accl. Lab, KEK, JAPAN
+This file is an offspring of the Lantz Project.
 
-    This file is an offspring of the Lantz Project.
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 from .usbtmc import USBRaw as USBRaw
 from .usbutil import find_devices, find_interfaces
diff --git a/pyvisa-py/protocols/usbtmc.py b/pyvisa-py/protocols/usbtmc.py
index e345bd4c..4863d9a3 100644
--- a/pyvisa-py/protocols/usbtmc.py
+++ b/pyvisa-py/protocols/usbtmc.py
@@ -1,19 +1,14 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.protocols.usbtmc
-    ~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-    Implements Session to control USBTMC instruments
+"""Implements Session to control USBTMC instruments
 
-    Loosely based on PyUSBTMC:python module to handle USB-TMC(Test and
-    Measurement class)　devices.
+Loosely based on PyUSBTMC:python module to handle USB-TMC(Test and
+Measurement class)　devices. by Noboru Yamamot, Accl. Lab, KEK, JAPAN
 
-    by Noboru Yamamot, Accl. Lab, KEK, JAPAN
+This file is an offspring of the Lantz Project.
 
-    This file is an offspring of the Lantz Project.
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import enum
 import struct
diff --git a/pyvisa-py/protocols/usbutil.py b/pyvisa-py/protocols/usbutil.py
index ad78463f..2b204906 100644
--- a/pyvisa-py/protocols/usbutil.py
+++ b/pyvisa-py/protocols/usbutil.py
@@ -1,18 +1,15 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.usb
-    ~~~~~~~~~~~~~
+"""Serial Session implementation using PyUSB.
 
-    Serial Session implementation using PyUSB.
+See the following link for more information about USB.
 
-    See the following link for more information about USB.
+http://www.beyondlogic.org/usbnutshell/usb5.shtml
 
-    http://www.beyondlogic.org/usbnutshell/usb5.shtml
+This file is an offspring of the Lantz Project.
 
-    This file is an offspring of the Lantz Project.
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 from fnmatch import fnmatch
 
diff --git a/pyvisa-py/protocols/vxi11.py b/pyvisa-py/protocols/vxi11.py
index 531e93c0..0df4d5b7 100644
--- a/pyvisa-py/protocols/vxi11.py
+++ b/pyvisa-py/protocols/vxi11.py
@@ -1,16 +1,13 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.protocols.vxi11
-    ~~~~~~~~~~~~~~~~~~~~~~~~~
+"""Implements a VX11 Session using Python Standard Library.
 
-    Implements a VX11 Session using Python Standard Library.
+Based on Python Sun RPC Demo and Alex Forencich python-vx11
 
-    Based on Python Sun RPC Demo and Alex Forencich python-vx11
+This file is an offspring of the Lantz project.
 
-    This file is an offspring of the Lantz project.
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import enum
 import socket
diff --git a/pyvisa-py/serial.py b/pyvisa-py/serial.py
index 9dac91b5..1408d49d 100644
--- a/pyvisa-py/serial.py
+++ b/pyvisa-py/serial.py
@@ -1,15 +1,20 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.serial
-    ~~~~~~~~~~~~~~~~
+"""Serial Session implementation using PySerial.
 
-    Serial Session implementation using PySerial.
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
+from typing import Any, List, Tuple
+
 from pyvisa import attributes, constants, logger
+from pyvisa.constants import (
+    BufferOperation,
+    ResourceAttribute,
+    SerialTermination,
+    StatusCode,
+)
 
 from . import common
 from .sessions import Session, UnknownAttribute
@@ -26,29 +31,23 @@
     raise
 
 
-def to_state(boolean_input):
-    """Convert a boolean input into a LineState value
-    """
+def to_state(boolean_input: bool) -> constants.LineState:
+    """Convert a boolean input into a LineState value."""
     if boolean_input:
         return constants.LineState.asserted
     return constants.LineState.unasserted
 
 
-StatusCode = constants.StatusCode
-SerialTermination = constants.SerialTermination
-
-
 @Session.register(constants.InterfaceType.asrl, "INSTR")
 class SerialSession(Session):
-    """A serial Session that uses PySerial to do the low level communication.
-    """
+    """A serial Session that uses PySerial to do the low level communication."""
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         return ["ASRL%s::INSTR" % port[0] for port in comports()]
 
     @classmethod
-    def get_low_level_info(cls):
+    def get_low_level_info(cls) -> str:
         try:
             ver = serial.VERSION
         except AttributeError:
@@ -56,11 +55,8 @@ def get_low_level_info(cls):
 
         return "via PySerial (%s)" % ver
 
-    def after_parsing(self):
-        if "mock" in self.parsed:
-            cls = self.parsed.mock
-        else:
-            cls = serial.Serial
+    def after_parsing(self) -> None:
+        cls = serial.Serial
 
         self.interface = cls(
             port=self.parsed.board, timeout=self.timeout, write_timeout=self.timeout
@@ -77,33 +73,41 @@ def after_parsing(self):
             attribute = getattr(constants, "VI_ATTR_" + name)
             self.attrs[attribute] = attributes.AttributesByID[attribute].default
 
-    def _get_timeout(self, attribute):
+    def _get_timeout(self, attribute: ResourceAttribute) -> Tuple[int, StatusCode]:
         if self.interface:
             self.timeout = self.interface.timeout
         return super(SerialSession, self)._get_timeout(attribute)
 
-    def _set_timeout(self, attribute, value):
+    def _set_timeout(self, attribute: ResourceAttribute, value: int) -> StatusCode:
         status = super(SerialSession, self)._set_timeout(attribute, value)
         if self.interface:
             self.interface.timeout = self.timeout
             self.interface.write_timeout = self.timeout
         return status
 
-    def close(self):
+    def close(self) -> None:
         self.interface.close()
 
-    def read(self, count):
+    def read(self, count: int) -> Tuple[bytes, StatusCode]:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: bytes, constants.StatusCode
-        """
+        Parameters
+        -----------
+        count : int
+            Number of bytes to be read.
 
-        end_in, _ = self.get_attribute(constants.VI_ATTR_ASRL_END_IN)
-        suppress_end_en, _ = self.get_attribute(constants.VI_ATTR_SUPPRESS_END_EN)
+        Returns
+        -------
+        bytes
+            Data read from the device
+        StatusCode
+            Return value of the library call.
+
+        """
+        end_in, _ = self.get_attribute(ResourceAttribute.asrl_end_in)
+        suppress_end_en, _ = self.get_attribute(ResourceAttribute.suppress_end_enabled)
 
         reader = lambda: self.interface.read(1)
 
@@ -115,7 +119,7 @@ def read(self, count):
             checker = lambda current: bool(common.last_int(current) & mask)
 
         elif end_in == SerialTermination.termination_char:
-            end_char, _ = self.get_attribute(constants.VI_ATTR_TERMCHAR)
+            end_char, _ = self.get_attribute(ResourceAttribute.termchar)
 
             checker = lambda current: common.last_int(current) == end_char
 
@@ -132,20 +136,28 @@ def read(self, count):
             serial.SerialTimeoutException,
         )
 
-    def write(self, data):
+    def write(self, data: bytes) -> Tuple[int, StatusCode]:
         """Writes data to device or interface synchronously.
 
         Corresponds to viWrite function of the VISA library.
 
-        :param data: data to be written.
-        :type data: bytes
-        :return: Number of bytes actually transferred, return value of the library call.
-        :rtype: int, VISAStatus
+        Parameters
+        ----------
+        data : bytes
+            Data to be written.
+
+        Returns
+        -------
+        int
+            Number of bytes actually transferred
+        StatusCode
+            Return value of the library call.
+
         """
         logger.debug("Serial.write %r" % data)
         # TODO: How to deal with VI_ATTR_TERMCHAR_EN
-        end_out, _ = self.get_attribute(constants.VI_ATTR_ASRL_END_OUT)
-        send_end, _ = self.get_attribute(constants.VI_ATTR_SEND_END_EN)
+        end_out, _ = self.get_attribute(ResourceAttribute.asrl_end_out)
+        send_end, _ = self.get_attribute(ResourceAttribute.send_end_enabled)
 
         try:
             # We need to wrap data in common.iter_bytes to Provide Python 2 and 3 compatibility
@@ -178,44 +190,67 @@ def write(self, data):
         except serial.SerialTimeoutException:
             return 0, StatusCode.error_timeout
 
-    def flush(self, mask):
-        """Flushes device buffers.
+    def flush(self, mask: BufferOperation) -> StatusCode:
+        """Flush the specified buffers.
+
+        The buffers can be associated with formatted I/O operations and/or
+        serial communication.
 
-        Corresponds to viFlush function of the VISA library. See:
-        https://pyvisa.readthedocs.io/en/latest/api/visalibrarybase.html?highlight=flush#pyvisa.highlevel.VisaLibraryBase.flush
-        for valid values of mask.
+        Corresponds to viFlush function of the VISA library.
+
+        Parameters
+        ----------
+        mask : constants.BufferOperation
+            Specifies the action to be taken with flushing the buffer.
+            The values can be combined using the | operator. However multiple
+            operations on a single buffer cannot be combined.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
 
-        :param mask: which buffers to clear.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
         """
         if (
-            mask & constants.VI_READ_BUF
-            or mask & constants.VI_READ_BUF_DISCARD
-            or mask & constants.VI_IO_IN_BUF
-            or mask & constants.VI_IO_IN_BUF_DISCARD
+            mask & BufferOperation.discard_read_buffer
+            or mask & BufferOperation.discard_read_buffer_no_io
+            or mask & BufferOperation.discard_receive_buffer
+            or mask & BufferOperation.discard_receive_buffer2
         ):
             self.interface.reset_input_buffer()
-        if mask & constants.VI_WRITE_BUF or mask & constants.VI_IO_OUT_BUF:
+        if (
+            mask & BufferOperation.flush_write_buffer
+            or mask & BufferOperation.flush_transmit_buffer
+        ):
             self.interface.flush()
         if (
-            mask & constants.VI_WRITE_BUF_DISCARD
-            or mask & constants.VI_IO_OUT_BUF_DISCARD
+            mask & BufferOperation.discard_write_buffer
+            or mask & BufferOperation.discard_transmit_buffer
         ):
             self.interface.reset_output_buffer()
 
         return StatusCode.success
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(
+        self, attribute: constants.ResourceAttribute
+    ) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource, return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
-        """
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Attribute for which the state query is made
 
+        Returns
+        -------
+        Any
+            State of the queried attribute for a specified resource
+        StatusCode
+            Return value of the library call.
+
+        """
         if attribute == constants.VI_ATTR_ASRL_ALLOW_TRANSMIT:
             raise NotImplementedError
 
@@ -302,17 +337,26 @@ def _get_attribute(self, attribute):
 
         raise UnknownAttribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: constants.ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
-        """
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Attribute for which the state is to be modified. (Attributes.*)
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
 
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
+        """
         if attribute == constants.VI_ATTR_ASRL_ALLOW_TRANSMIT:
             raise NotImplementedError
 
diff --git a/pyvisa-py/sessions.py b/pyvisa-py/sessions.py
index 3bf7d4ed..aa73065f 100644
--- a/pyvisa-py/sessions.py
+++ b/pyvisa-py/sessions.py
@@ -1,29 +1,30 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.session
-    ~~~~~~~~~~~~~~~~~
+"""Base Session class.
 
-    Base Session class.
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import abc
 import time
+from typing import Any, Callable, Dict, Iterator, Optional, Tuple, Type, TypeVar
 
 from pyvisa import attributes, constants, logger, rname
+from pyvisa.constants import ResourceAttribute, StatusCode
+from pyvisa.typing import VISARMSession
 
-from . import common
-
-StatusCode = constants.StatusCode
+#: Type var used when typing register.
+T = TypeVar("T", bounds="Session")
 
 
 class UnknownAttribute(Exception):
-    def __init__(self, attribute):
+    """Custom exception signaling a VISA attribute is not supported."""
+
+    def __init__(self, attribute: constants.ResourceAttribute) -> None:
         self.attribute = attribute
 
-    def __str__(self):
+    def __str__(self) -> str:
         attr = self.attribute
         if isinstance(attr, int):
             try:
@@ -43,69 +44,121 @@ class Session(metaclass=abc.ABCMeta):
 
     Just makes sure that common methods are defined and information is stored.
 
-    :param resource_manager_session: The session handle of the parent Resource
-        Manager
-    :param resource_name: The resource name.
-    :param parsed: the parsed resource name (optional).
-                   If not provided, the resource_name will be parsed.
+    Parameters
+    ----------
+    resource_manager_session : VISARMSession
+        Session handle of the parent Resource Manager
+    resource_name : str
+        Name of the resource this session is communicating with
+    parsed : rname.ResourceName, optional
+        Parsed representation of the resource name. The default is False meaning
+        that the provided resource name will be parsed.
+
     """
 
     @abc.abstractmethod
-    def _get_attribute(self, attribute):
+    def _get_attribute(
+        self, attribute: constants.ResourceAttribute
+    ) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource,
-            return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Resource attribute for which the state query is made
+
+        Returns
+        -------
+        Any
+            State of the queried attribute for a specified resource
+        constants.StatusCode
+            Return value of the library call.
+
         """
 
     @abc.abstractmethod
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: constants.ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Set the attribute_state value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made.
-        :param attribute_state: value.
-        :return: The return value of the library call.
-        :rtype: VISAStatus
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Resource attribute for which the state query is made.
+        attribute_state : Any
+            Value to which to set the attribute.
+
+        Returns
+        -------
+        StatusCode
+            The return value of the library call.
+
         """
 
     @abc.abstractmethod
-    def close(self):
-        """Close the session. Use it to do final clean ups.
+    def close(self) -> None:
+        """Close the session.
+
+        Use it to do final clean ups.
+
         """
 
+    #: Session handle of the parent Resource Manager
+    resource_manager_session: VISARMSession
+
+    #: Name of the resource this session is communicating with
+    resource_name: str
+
+    #: Parsed representation of the resource name.
+    parsed: rname.ResourceName
+
+    #: Session type as (Interface Type, Resource Class)
+    session_type: Optional[Tuple[constants.InterfaceType, str]] = None
+
+    #: Timeout in seconds to use when opening the resource.
+    open_timeout: float
+
+    #: Value of the timeout in seconds used for general operation
+    timeout: Optional[float]
+
+    #: Used as a place holder for the object doing the lowlevel communication.
+    interface: Any
+
+    #: Used for attributes not handled by the underlying interface.
+    #: Values are get or set automatically by get_attribute and set_attribute
+    attrs: Dict[constants.ResourceAttribute, Any]
+
     #: Maps (Interface Type, Resource Class) to Python class encapsulating that
     #: resource.
     #: dict[(Interface Type, Resource Class) , Session]
-    _session_classes = dict()
-
-    #: Session type as (Interface Type, Resource Class)
-    session_type = None
+    _session_classes: Dict[
+        Tuple[constants.InterfaceType, str], Type["Session"]
+    ] = dict()
 
     @classmethod
-    def get_low_level_info(cls):
+    def get_low_level_info(cls) -> str:
+        """Get info about the backend used by the session."""
         return ""
 
     @classmethod
-    def iter_valid_session_classes(cls):
-        """Yield (Interface Type, Resource Class), Session class pair for
-        valid sessions classes.
-        """
-
+    def iter_valid_session_classes(
+        cls,
+    ) -> Iterator[Tuple[constants.InterfaceType, str], Type["Session"]]:
+        """Iterator over valid sessions classes infos."""
         for key, val in cls._session_classes.items():
             if issubclass(val, Session):
                 yield key, val
 
     @classmethod
-    def iter_session_classes_issues(cls):
-        """Yield (Interface Type, Resource Class), Issues class pair for
-        invalid sessions classes (i.e. those with import errors).
-        """
+    def iter_session_classes_issues(
+        cls,
+    ) -> Iterator[Tuple[constants.InterfaceType, str], str]:
+        """Iterator over invalid sessions classes (i.e. those with import errors)."""
         for key, val in cls._session_classes.items():
             try:
                 yield key, getattr(val, "session_issue")
@@ -113,12 +166,23 @@ def iter_session_classes_issues(cls):
                 pass
 
     @classmethod
-    def get_session_class(cls, interface_type, resource_class):
-        """Return the session class for a given interface type and resource class.
+    def get_session_class(
+        cls, interface_type: constants.InterfaceType, resource_class: str
+    ) -> Type["Session"]:
+        """Get the session class for a given interface type and resource class.
+
+        Parameters
+        ----------
+        interface_type : constants.InterfaceType
+            Type of interface.
+        resource_class : str
+            Class of resource.
+
+        Returns
+        -------
+        Sessions
+            Session subclass the most appropriate for the resource.
 
-        :type interface_type: constants.InterfaceType
-        :type resource_class: str
-        :return: Session
         """
         try:
             return cls._session_classes[(interface_type, resource_class)]
@@ -128,11 +192,23 @@ def get_session_class(cls, interface_type, resource_class):
             )
 
     @classmethod
-    def register(cls, interface_type, resource_class):
+    def register(
+        cls, interface_type: constants.InterfaceType, resource_class: str
+    ) -> Callable[[T], T]:
         """Register a session class for a given interface type and resource class.
 
-        :type interface_type: constants.InterfaceType
-        :type resource_class: str
+        Parameters
+        ----------
+        interface_type : constants.InterfaceType
+            Type of interface.
+        resource_class : str
+            Class of the resource
+
+        Returns
+        -------
+        Callable[[T], T]
+            Decorator function to register a session subclass.
+
         """
 
         def _internal(python_class):
@@ -150,23 +226,37 @@ def _internal(python_class):
         return _internal
 
     @classmethod
-    def register_unavailable(cls, interface_type, resource_class, msg):
-        """Register an unavailable session class for a given interface type and
-        resource class.
+    def register_unavailable(
+        cls, interface_type: constants.InterfaceType, resource_class: str, msg: str
+    ) -> Type[object]:
+        """Register that no session class exists.
+
+        This creates a fake session that will raise a ValueError if called.
+
+        Parameters
+        ----------
+        interface_type : constants.InterfaceType
+            Type of interface.
+        resource_class : str
+            Class of the resource
+        msg : str
+            Message detailing why no session class exists for this particular
+            interface type, resource class pair.
+
+        Returns
+        -------
+        Type[object]
+            Fake session.
 
-        raising a ValueError if called.
-
-        :type interface_type: constants.InterfaceType
-        :type resource_class: str
         """
-        # noinspection PyUnusedLocal
+
         class _internal(object):
-            session_issue = msg
 
-            def __init__(self, *args, **kwargs):
-                raise ValueError(msg)
+            #: Message detailing why no session is available.
+            session_issue: str = msg
 
-        _internal.session_issue = msg
+            def __init__(self, *args, **kwargs) -> None:
+                raise ValueError(msg)
 
         if (interface_type, resource_class) in cls._session_classes:
             logger.warning(
@@ -178,20 +268,19 @@ def __init__(self, *args, **kwargs):
         cls._session_classes[(interface_type, resource_class)] = _internal
 
     def __init__(
-        self, resource_manager_session, resource_name, parsed=None, open_timeout=None
-    ):
-        if isinstance(resource_name, common.MockInterface):
-            parsed = rname.parse_resource_name(resource_name.resource_name)
-            parsed["mock"] = resource_name
-
-        elif parsed is None:
+        self,
+        resource_manager_session: VISARMSession,
+        resource_name: str,
+        parsed: Optional[rname.ResourceName] = None,
+        open_timeout: Optional[int] = None,
+    ) -> None:
+        if parsed is None:
             parsed = rname.parse_resource_name(resource_name)
 
         self.parsed = parsed
         self.open_timeout = open_timeout
 
-        #: Used as a place holder for the object doing the lowlevel
-        #: communication.
+        #: Used as a place holder for the object doing the lowlevel communication.
         self.interface = None
 
         #: Used for attributes not handled by the underlying interface.
@@ -199,26 +288,25 @@ def __init__(
         #: set_attribute
         #: Add your own by overriding after_parsing.
         self.attrs = {
-            constants.VI_ATTR_RM_SESSION: resource_manager_session,
-            constants.VI_ATTR_RSRC_NAME: str(parsed),
-            constants.VI_ATTR_RSRC_CLASS: parsed.resource_class,
-            constants.VI_ATTR_INTF_TYPE: parsed.interface_type,
-            constants.VI_ATTR_TMO_VALUE: (self._get_timeout, self._set_timeout),
+            ResourceAttribute.resource_manager_session: resource_manager_session,
+            ResourceAttribute.resource_name: str(parsed),
+            ResourceAttribute.resource_class: parsed.resource_class,
+            ResourceAttribute.interface_type: parsed.interface_type,
+            ResourceAttribute.timeout_value: (self._get_timeout, self._set_timeout),
         }
 
         #: Timeout expressed in second or None for the absence of a timeout.
-        #: The default value is set when calling
-        #: self.set_attribute(attr, default_timeout)
+        #: The default value is set when calling self.set_attribute(attr, default_timeout)
         self.timeout = None
 
         #: Set the default timeout from constants
-        attr = constants.VI_ATTR_TMO_VALUE
+        attr = ResourceAttribute.timeout_value
         default_timeout = attributes.AttributesByID[attr].default
         self.set_attribute(attr, default_timeout)
 
         self.after_parsing()
 
-    def after_parsing(self):
+    def after_parsing(self) -> None:
         """Override this method to provide custom initialization code, to be
         called after the resourcename is properly parsed
 
@@ -242,7 +330,7 @@ def after_parsing(self):
         Getter has same signature as see Session._get_attribute and setter has
         same signature as see Session._set_attribute. (It is possible to
         register also see Session._get_attribute and see Session._set_attribute
-        as getter/setter). Getter and Setter are registered as tupple.
+        as getter/setter). Getter and Setter are registered as tuple.
         For readwrite attribute:
         `    self.attrs[constants.VI_ATTR_<NAME>] = (<getter_name>,
                                                      <setter_name>)`
@@ -252,151 +340,243 @@ def after_parsing(self):
         Session._set_attribute
         `    self.attrs[constants.VI_ATTR_<NAME>] = (self._get_attribute,
                                                      self._set_attribute)`
+
         """
         pass
 
-    def clear(self):
+    def clear(self) -> StatusCode:
         """Clears a device.
 
         Corresponds to viClear function of the VISA library.
 
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         return StatusCode.error_nonsupported_operation
 
-    def flush(self, mask):
-        """Flushes device buffers.
+    def flush(self, mask: constants.BufferOperation) -> StatusCode:
+        """Flush the specified buffers.
 
-        Corresponds to viFlush function of the VISA library. See:
-        https://pyvisa.readthedocs.io/en/latest/api/visalibrarybase.html?highlight=flush#pyvisa.highlevel.VisaLibraryBase.flush
-        for valid values of mask.
+        The buffers can be associated with formatted I/O operations and/or
+        serial communication.
+
+        Corresponds to viFlush function of the VISA library.
+
+        Parameters
+        ----------
+        mask : constants.BufferOperation
+            Specifies the action to be taken with flushing the buffer.
+            The values can be combined using the | operator. However multiple
+            operations on a single buffer cannot be combined.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
 
-        :param mask: which buffers to clear.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
         """
         raise NotImplementedError
 
-    def read_stb(self):
+    def read_stb(self) -> Tuple[int, StatusCode]:
         """Reads a status byte of the service request.
 
         Corresponds to viReadSTB function of the VISA library.
 
-        :return: Service request status byte, return value of the library call.
-        :rtype: int, :class:`pyvisa.constants.StatusCode`
+        Returns
+        -------
+        int
+            Service request status byte
+        StatusCode
+            Return value of the library call.
+
         """
         return 0, StatusCode.error_nonsupported_operation
 
-    def lock(self, session, lock_type, timeout, requested_key=None):
+    def lock(
+        self,
+        lock_type: constants.Lock,
+        timeout: int,
+        requested_key: Optional[str] = None,
+    ):
         """Establishes an access mode to the specified resources.
 
         Corresponds to viLock function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :param lock_type: Specifies the type of lock requested, either Constants.EXCLUSIVE_LOCK or Constants.SHARED_LOCK.
-        :param timeout: Absolute time period (in milliseconds) that a resource waits to get unlocked by the
-                        locking session before returning an error.
-        :param requested_key: This parameter is not used and should be set to VI_NULL when lockType is VI_EXCLUSIVE_LOCK.
-        :return: access_key that can then be passed to other sessions to share the lock, return value of the library call.
-        :rtype: str, :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        lock_type : constants.Lock
+            Specifies the type of lock requested.
+        timeout : int
+            Absolute time period (in milliseconds) that a resource waits to get
+            unlocked by the locking session before returning an error.
+        requested_key : Optional[str], optional
+            Requested locking key in the case of a shared lock. For an exclusive
+            lock it should be None.
+
+        Returns
+        -------
+        Optional[str]
+            Key that can then be passed to other sessions to share the lock, or
+            None for an exclusive lock.
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return "", StatusCode.error_nonsupported_operation
 
-    def unlock(self, session):
+    def unlock(self) -> StatusCode:
         """Relinquishes a lock for the specified resource.
 
         Corresponds to viUnlock function of the VISA library.
 
-        :param session: Unique logical identifier to a session.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         return StatusCode.error_nonsupported_operation
 
-    def gpib_command(self, command_byte):
-        """Write GPIB command byte on the bus.
+    def gpib_command(self, command_byte: bytes) -> Tuple[int, StatusCode]:
+        """Write GPIB command bytes on the bus.
 
         Corresponds to viGpibCommand function of the VISA library.
-        See: https://linux-gpib.sourceforge.io/doc_html/gpib-protocol.html#REFERENCE-COMMAND-BYTES
 
-        :param command_byte: command byte to send
-        :type command_byte: int, must be [0 255]
-        :return: Number of written bytes, return value of the library call.
-        :rtype: int, :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        data : bytes
+            Data to write.
+
+        Returns
+        -------
+        int
+            Number of written bytes
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return 0, StatusCode.error_nonsupported_operation
 
-    def assert_trigger(self, protocol):
-        """Asserts software or hardware trigger.
+    def assert_trigger(self, protocol: constants.TriggerProtocol) -> StatusCode:
+        """Assert software or hardware trigger.
 
         Corresponds to viAssertTrigger function of the VISA library.
 
-        :param protocol: Trigger protocol to use during assertion. (Constants.PROT*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        protocol : constants.TriggerProtocol
+            Trigger protocol to use during assertion.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         raise NotImplementedError
 
-    def gpib_send_ifc(self):
+    def gpib_send_ifc(self) -> StatusCode:
         """Pulse the interface clear line (IFC) for at least 100 microseconds.
 
         Corresponds to viGpibSendIFC function of the VISA library.
 
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return StatusCode.error_nonsupported_operation
 
-    def gpib_control_ren(self, mode):
-        """Controls the state of the GPIB Remote Enable (REN) interface line, and optionally the remote/local
-        state of the device.
+    def gpib_control_ren(
+        self, mode: constants.RENLineOperation
+    ) -> constants.StatusCode:
+        """Controls the state of the GPIB Remote Enable (REN) interface line.
+
+        Optionally the remote/local state of the device can also be set.
 
         Corresponds to viGpibControlREN function of the VISA library.
 
-        :param mode: Specifies the state of the REN line and optionally the device remote/local state.
-                     (Constants.VI_GPIB_REN*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        mode : constants.RENLineOperation
+            State of the REN line and optionally the device remote/local state.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return StatusCode.error_nonsupported_operation
 
-    def gpib_control_atn(self, mode):
+    def gpib_control_atn(self, mode: constants.ATNLineOperation) -> StatusCode:
         """Specifies the state of the ATN line and the local active controller state.
 
         Corresponds to viGpibControlATN function of the VISA library.
 
-        :param mode: Specifies the state of the ATN line and optionally the local active controller state.
-                     (Constants.VI_GPIB_ATN*)
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        mode : constants.ATNLineOperation
+            State of the ATN line and optionally the local active controller state.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return StatusCode.error_nonsupported_operation
 
-    def gpib_pass_control(self, primary_address, secondary_address):
-        """Tell the GPIB device at the specified address to become controller in charge (CIC).
+    def gpib_pass_control(
+        self, primary_address: int, secondary_address: int
+    ) -> StatusCode:
+        """Tell a GPIB device to become controller in charge (CIC).
 
         Corresponds to viGpibPassControl function of the VISA library.
 
-        :param primary_address: Primary address of the GPIB device to which you want to pass control.
-        :param secondary_address: Secondary address of the targeted GPIB device.
-                                  If the targeted device does not have a secondary address,
-                                  this parameter should contain the value Constants.VI_NO_SEC_ADDR.
-        :return: return value of the library call.
-        :rtype: :class:`pyvisa.constants.StatusCode`
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        primary_address : int
+            Primary address of the GPIB device to which you want to pass control.
+        secondary_address : int
+            Secondary address of the targeted GPIB device.
+            If the targeted device does not have a secondary address, this parameter
+            should contain the value Constants.VI_NO_SEC_ADDR.
+
+        Returns
+        -------
+        constants.StatusCode
+            Return value of the library call.
+
         """
         return StatusCode.error_nonsupported_operation
 
-    def get_attribute(self, attribute):
+    def get_attribute(self, attribute: ResourceAttribute) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Does a few checks before and calls before dispatching to
         `_get_attribute`.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource,
-            return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Resource attribute for which the state query is made
+
+        Returns
+        -------
+        Any
+            The state of the queried attribute for a specified resource.
+        StatusCode
+            Return value of the library call.
+
         """
 
         # Check if the attribute value is defined.
@@ -435,19 +615,28 @@ def get_attribute(self, attribute):
             logger.exception(str(e))
             return 0, StatusCode.error_nonsupported_attribute
 
-    def set_attribute(self, attribute, attribute_state):
+    def set_attribute(
+        self, attribute: ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Set the attribute_state value for a given VISA attribute for this
         session.
 
         Does a few checks before and calls before dispatching to
         `_gst_attribute`.
 
-        :param attribute: Resource attribute for which the state query is made.
-        :param attribute_state: value.
-        :return: The return value of the library call.
-        :rtype: VISAStatus
-        """
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Resource attribute for which the state query is made.
+        attribute_state : Any
+            Value.
 
+        Returns
+        -------
+        StatusCode
+            The return value of the library call.
+
+        """
         # Check if the attribute value is defined.
         try:
             attr = attributes.AttributesByID[attribute]
@@ -494,38 +683,43 @@ def set_attribute(self, attribute, attribute_state):
 
     def _read(
         self,
-        reader,
-        count,
-        end_indicator_checker,
-        suppress_end_en,
-        termination_char,
-        termination_char_en,
-        timeout_exception,
-    ):
+        reader: Callable[[], bytes],
+        count: int,
+        end_indicator_checker: Callable[[bytes], bool],
+        suppress_end_en: bool,
+        termination_char: str,
+        termination_char_en: bool,
+        timeout_exception: Type[Exception],
+    ) -> Tuple[bytes, StatusCode]:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param reader: Function to read one or more bytes.
-        :type reader: () -> bytes
-        :param count: Number of bytes to be read.
-        :type count: int
-        :param end_indicator_checker: Function to check if the message is
-            complete.
-        :type end_indicator_checker: (bytes) -> boolean
-        :param suppress_end_en: suppress end.
-        :type suppress_end_en: bool
-        :param termination_char: Stop reading if this character is received.
-        :type suppress_end_en: int or str
-        :param termination_char_en: termination char enabled.
-        :type termination_char_en: boolean
-        :param: timeout_exception: Exception to capture time out for the given
-            interface.
-        :type: Exception
-        :return: data read, return value of the library call.
-        :rtype: bytes, constants.StatusCode
-        """
+        Parameters
+        ----------
+        reader : Callable[[], bytes]
+            Function to read one or more bytes.
+        count : int
+            Number of bytes to be read.
+        end_indicator_checker : Callable[[bytes], bool]
+            Function to check if the message is complete.
+        suppress_end_en : bool
+            Suppress end.
+        termination_char : str
+            Stop reading if this character is received.
+        termination_char_en : bool
+            Is termination char enabled.
+        timeout_exception : Type[Exception]
+            Exception to capture time out for the given interface.
+
+        Returns
+        -------
+        bytes
+            Data read from the resource.
+        StatusCode
+            Return value of the library call.
 
+        """
         # NOTE: Some interfaces return not only a single byte but a complete
         # block for each read therefore we must handle the case that the
         # termination character is in the middle of the  block or that the
@@ -569,7 +763,7 @@ def _read(
             if finish_time and time.time() > finish_time:
                 return bytes(out), StatusCode.error_timeout
 
-    def _get_timeout(self, attribute):
+    def _get_timeout(self, attribute: ResourceAttribute) -> Tuple[int, StatusCode]:
         """ Returns timeout calculated value from python way to VI_ way
 
         In VISA, the timeout is expressed in milliseconds or using the
@@ -587,7 +781,7 @@ def _get_timeout(self, attribute):
             ret_value = int(self.timeout * 1000.0)
         return ret_value, StatusCode.success
 
-    def _set_timeout(self, attribute, value):
+    def _set_timeout(self, attribute: ResourceAttribute, value: Optional[int]):
         """ Sets timeout calculated value from python way to VI_ way
 
         In VISA, the timeout is expressed in milliseconds or using the
diff --git a/pyvisa-py/tcpip.py b/pyvisa-py/tcpip.py
index 12d8c517..7db2ed20 100644
--- a/pyvisa-py/tcpip.py
+++ b/pyvisa-py/tcpip.py
@@ -1,27 +1,24 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.tcpip
-    ~~~~~~~~~~~~~~~
+"""TCPIP Session implementation using Python Standard library.
 
-    TCPIP Session implementation using Python Standard library.
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import random
 import select
 import socket
 import time
+from typing import Any, List, Optional, Tuple
 
 from pyvisa import attributes, constants, errors
+from pyvisa.constants import ResourceAttribute, StatusCode
 
 from . import common
 from .protocols import rpc, vxi11
 from .sessions import Session, UnknownAttribute
 
-StatusCode = constants.StatusCode
-
 # Conversion between VXI11 error codes and VISA status
 # TODO this is so far a best guess, in particular 6 and 29 are likely wrong
 VXI11_ERRORS_TO_VISA = {
@@ -44,17 +41,26 @@
 
 @Session.register(constants.InterfaceType.tcpip, "INSTR")
 class TCPIPInstrSession(Session):
-    """A TCPIP Session that uses the network standard library to do the low
-    level communication using VXI-11
+    """A TCPIP Session built on socket standard library using VXI-11 protocol."""
 
-    """
+    #: Maximum size of a chunk of data in bytes.
+    max_recv_size: int
+
+    #: Time to wait before erroring with a timeout when trying to acquire a lock
+    lock_timeout: int
+
+    #: Unique ID of the client used to authenticate messages.
+    client_id: int
+
+    #: ID of the link used for VXI-11 communication
+    link: int
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         # TODO: is there a way to get this?
         return []
 
-    def after_parsing(self):
+    def after_parsing(self) -> None:
         # TODO: board_number not handled
         # vx11 expect all timeouts to be expressed in ms and should be integers
         try:
@@ -64,7 +70,6 @@ def after_parsing(self):
         except rpc.RPCError:
             raise errors.VisaIOError(constants.VI_ERROR_RSRC_NFOUND)
 
-        self.max_recv_size = 1024
         self.lock_timeout = 10000
         self.client_id = random.getrandbits(31)
 
@@ -82,7 +87,7 @@ def after_parsing(self):
             attribute = getattr(constants, "VI_ATTR_" + name)
             self.attrs[attribute] = attributes.AttributesByID[attribute].default
 
-    def close(self):
+    def close(self) -> None:
         try:
             self.interface.destroy_link(self.link)
         except (errors.VisaIOError, socket.error, rpc.RPCError) as e:
@@ -92,14 +97,23 @@ def close(self):
         self.link = None
         self.interface = None
 
-    def read(self, count):
+    def read(self, count: int) -> Tuple[bytes, StatusCode]:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: bytes, VISAStatus
+        Parameters
+        ----------
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Data read
+        StatusCode
+            Return value of the library call.
+
         """
         if count < self.max_recv_size:
             chunk_length = count
@@ -146,18 +160,24 @@ def read(self, count):
 
         return bytes(read_data), status
 
-    def write(self, data):
+    def write(self, data: bytes) -> Tuple[int, StatusCode]:
         """Writes data to device or interface synchronously.
 
         Corresponds to viWrite function of the VISA library.
 
-        :param data: data to be written.
-        :type data: str
-        :return: Number of bytes actually transferred, return value of the
-            library call.
-        :rtype: int, VISAStatus
-        """
+        Parameters
+        ----------
+        data : bytes
+            Data to be written.
 
+        Returns
+        -------
+        int
+            Number of bytes actually transferred
+        StatusCode
+            Return value of the library call.
+
+        """
         send_end, _ = self.get_attribute(constants.VI_ATTR_SEND_END_EN)
         chunk_size = 1024
 
@@ -190,17 +210,24 @@ def write(self, data):
         except vxi11.Vxi11Error:
             return 0, StatusCode.error_timeout
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(self, attribute: ResourceAttribute) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource,
-            return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
-        """
+        Parameters
+        ----------
+        attribute :
+            Resource attribute for which the state query is made
 
+        Returns
+        -------
+        Any
+            The state of the queried attribute for a specified resource
+        StatusCode
+            Return value of the library call.
+
+        """
         if attribute == constants.VI_ATTR_TCPIP_ADDR:
             return self.parsed.host_address, StatusCode.success
 
@@ -224,85 +251,114 @@ def _get_attribute(self, attribute):
 
         raise UnknownAttribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified.
-            (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the
-            specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
-        """
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+             Attribute for which the state is to be modified.
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
 
+        """
         raise UnknownAttribute(attribute)
 
-    def assert_trigger(self, protocol):
+    def assert_trigger(self, protocol: constants.TriggerProtocol):
         """Asserts software or hardware trigger.
 
         Corresponds to viAssertTrigger function of the VISA library.
 
-        :param protocol: Trigger protocol to use during assertion.
-            (Constants.PROT*)
-        :return: return value of the library call.
-        :rtype: VISAStatus
-        """
+        Parameters
+        ----------
+        protocol : constants.TriggerProtocol
+            Trigger protocol to use during assertion. Only default is supported.
+
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
 
+        """
+        # XXX make this nicer (either validate protocol or pass it)
         error = self.interface.device_trigger(
-            self.link, 0, self.lock_timeout, self._io_timeout
+            self.link, 0, self.lock_timeout, self._io_timeout,
         )
 
         return VXI11_ERRORS_TO_VISA[error]
 
-    def clear(self):
+    def clear(self) -> StatusCode:
         """Clears a device.
 
         Corresponds to viClear function of the VISA library.
 
-        :return: return value of the library call.
-        :rtype: VISAStatus
         """
-
         error = self.interface.device_clear(
             self.link, 0, self.lock_timeout, self._io_timeout
         )
 
         return VXI11_ERRORS_TO_VISA[error]
 
-    def read_stb(self):
+    def read_stb(self) -> Tuple[int, StatusCode]:
         """Reads a status byte of the service request.
 
         Corresponds to viReadSTB function of the VISA library.
 
-        :return: Service request status byte, return value of the library call.
-        :rtype: int, :class:`pyvisa.constants.StatusCode`
-        """
+        Returns
+        -------
+        int
+            Service request status byte
+        StatusCode
+            Return value of the library call.
 
+        """
         error, stb = self.interface.device_read_stb(
             self.link, 0, self.lock_timeout, self._io_timeout
         )
 
         return stb, VXI11_ERRORS_TO_VISA[error]
 
-    def lock(self, lock_type, timeout, requested_key=None):
+    def lock(
+        self,
+        lock_type: constants.Lock,
+        timeout: int,
+        requested_key: Optional[str] = None,
+    ) -> Tuple[str, constants.StatusCode]:
         """Establishes an access mode to the specified resources.
 
         Corresponds to viLock function of the VISA library.
 
-        :param lock_type: Specifies the type of lock requested, either
-            Constants.EXCLUSIVE_LOCK or Constants.SHARED_LOCK.
-        :param timeout: Absolute time period (in milliseconds) that a resource
-            waits to get unlocked by the locking session before returning an
-            error.
-        :param requested_key: This parameter is not used and should be set to
-            VI_NULL when lockType is VI_EXCLUSIVE_LOCK.
-        :return: access_key that can then be passed to other sessions to share
-            the lock, return value of the library call.
-        :rtype: str, VISAStatus
-        """
+        Parameters
+        ----------
+        session : VISASession
+            Unique logical identifier to a session.
+        lock_type : constants.Lock
+            Specifies the type of lock requested.
+        timeout : int
+            Absolute time period (in milliseconds) that a resource waits to get
+            unlocked by the locking session before returning an error.
+        requested_key : Optional[str], optional
+            Requested locking key in the case of a shared lock. For an exclusive
+            lock it should be None.
+
+        Returns
+        -------
+        Optional[str]
+            Key that can then be passed to other sessions to share the lock, or
+            None for an exclusive lock.
+        StatusCode
+            Return value of the library call.
 
+        """
         #  TODO: lock type not implemented
         flags = 0
 
@@ -310,19 +366,24 @@ def lock(self, lock_type, timeout, requested_key=None):
 
         return "", VXI11_ERRORS_TO_VISA[error]
 
-    def unlock(self):
-        """Relinquishes a lock for the specified resource.
+    def unlock(self) -> constants.StatusCode:
+        """Relinquish a lock for the specified resource.
 
         Corresponds to viUnlock function of the VISA library.
 
-        :return: return value of the library call.
-        :rtype: VISAStatus
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
         """
         error = self.interface.device_unlock(self.link)
 
         return VXI11_ERRORS_TO_VISA[error]
 
-    def _set_timeout(self, attribute, value):
+    def _set_timeout(
+        self, attribute: ResourceAttribute, value: Optional[int]
+    ) -> StatusCode:
         """ Sets timeout calculated value from python way to VI_ way
 
         """
@@ -353,12 +414,15 @@ class TCPIPSocketSession(Session):
     # timeout. The absolute minimum is 1 ms as a consequence.
     # This is valid for connect and read operations
 
+    #: Maximum size of a chunk of data in bytes.
+    max_recv_size: int
+
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         # TODO: is there a way to get this?
         return []
 
-    def after_parsing(self):
+    def after_parsing(self) -> None:
         # TODO: board_number not handled
 
         ret_status = self._connect()
@@ -390,7 +454,7 @@ def after_parsing(self):
             attribute = getattr(constants, "VI_ATTR_" + name)
             self.attrs[attribute] = attributes.AttributesByID[attribute].default
 
-    def _connect(self):
+    def _connect(self) -> None:
         timeout = self.open_timeout / 1000.0 if self.open_timeout else 10.0
         try:
             self.interface = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
@@ -424,18 +488,27 @@ def _connect(self):
             # min_select_timeout
             select_timout = max(select_timout / 2.0, min_select_timeout)
 
-    def close(self):
+    def close(self) -> None:
         self.interface.close()
         self.interface = None
 
-    def read(self, count):
+    def read(self, count: int) -> Tuple[bytes, StatusCode]:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: bytes, VISAStatus
+         Parameters
+        -----------
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Data read from the device
+        StatusCode
+            Return value of the library call.
+
         """
         if count < self.max_recv_size:
             chunk_length = count
@@ -511,18 +584,24 @@ def read(self, count):
                 # min_select_timeout
                 select_timout = max(select_timout / 2.0, min_select_timeout)
 
-    def write(self, data):
+    def write(self, data: bytes) -> Tuple[int, StatusCode]:
         """Writes data to device or interface synchronously.
 
         Corresponds to viWrite function of the VISA library.
 
-        :param data: data to be written.
-        :type data: str
-        :return: Number of bytes actually transferred, return value of the
-            library call.
-        :rtype: int, VISAStatus
-        """
+        Parameters
+        ----------
+        data : bytes
+            Data to be written.
+
+        Returns
+        -------
+        int
+            Number of bytes actually transferred
+        StatusCode
+            Return value of the library call.
 
+        """
         chunk_size = 4096
 
         num = sz = len(data)
@@ -548,63 +627,90 @@ def write(self, data):
 
         return offset, StatusCode.success
 
-    def _get_tcpip_nodelay(self, attribute):
+    def _get_tcpip_nodelay(
+        self, attribute: ResourceAttribute
+    ) -> Tuple[constants.VisaBoolean, StatusCode]:
         if self.interface:
             value = self.interface.getsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY)
             return (
-                constants.VI_TRUE if value == 1 else constants.VI_FALSE,
+                constants.VisaBoolean.true
+                if value == 1
+                else constants.VisaBoolean.false,
                 StatusCode.success,
             )
-        return 0, StatusCode.error_nonsupported_attribute
+        return constants.VisaBoolean.false, StatusCode.error_nonsupported_attribute
 
-    def _set_tcpip_nodelay(self, attribute, attribute_state):
+    def _set_tcpip_nodelay(
+        self, attribute: ResourceAttribute, attribute_state: bool
+    ) -> StatusCode:
         if self.interface:
             self.interface.setsockopt(
                 socket.IPPROTO_TCP, socket.TCP_NODELAY, 1 if attribute_state else 0
             )
             return StatusCode.success
-        return 0, StatusCode.error_nonsupported_attribute
+        return StatusCode.error_nonsupported_attribute
 
-    def _get_tcpip_keepalive(self, attribute):
+    def _get_tcpip_keepalive(
+        self, attribute: ResourceAttribute
+    ) -> Tuple[constants.VisaBoolean, StatusCode]:
         if self.interface:
             value = self.interface.getsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE)
             return (
-                constants.VI_TRUE if value == 1 else constants.VI_FALSE,
+                constants.VisaBoolean.true
+                if value == 1
+                else constants.VisaBoolean.false,
                 StatusCode.success,
             )
-        return 0, StatusCode.error_nonsupported_attribute
+        return constants.VisaBoolean.false, StatusCode.error_nonsupported_attribute
 
-    def _set_tcpip_keepalive(self, attribute, attribute_state):
+    def _set_tcpip_keepalive(
+        self, attribute: ResourceAttribute, attribute_state: bool
+    ) -> StatusCode:
         if self.interface:
             self.interface.setsockopt(
                 socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1 if attribute_state else 0
             )
             return StatusCode.success
-        return 0, StatusCode.error_nonsupported_attribute
+        return StatusCode.error_nonsupported_attribute
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(self, attribute: ResourceAttribute) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource,
-            return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Attribute for which the state query is made
+
+        Returns
+        -------
+        Any
+            State of the queried attribute for a specified resource
+        StatusCode
+            Return value of the library call.
+
         """
         raise UnknownAttribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified.
-            (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the
-            specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
-        """
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Attribute for which the state is to be modified. (Attributes.*)
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
 
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
+
+        """
         raise UnknownAttribute(attribute)
diff --git a/pyvisa-py/usb.py b/pyvisa-py/usb.py
index 40decddd..976ac625 100644
--- a/pyvisa-py/usb.py
+++ b/pyvisa-py/usb.py
@@ -1,17 +1,16 @@
 # -*- coding: utf-8 -*-
-"""
-    pyvisa-py.usb
-    ~~~~~~~~~~~~~
+"""Serial Session implementation using PyUSB.
 
-    Serial Session implementation using PyUSB.
 
+:copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
+:license: MIT, see LICENSE for more details.
 
-    :copyright: 2014-2020 by PyVISA-py Authors, see AUTHORS for more details.
-    :license: MIT, see LICENSE for more details.
 """
 import errno
+from typing import Any, List, Tuple
 
 from pyvisa import attributes, constants
+from pyvisa.constants import ResourceAttribute, StatusCode
 
 from .common import logger
 from .sessions import Session, UnknownAttribute
@@ -41,9 +40,6 @@
     raise
 
 
-StatusCode = constants.StatusCode
-
-
 class USBTimeoutException(Exception):
     """Exception used internally to indicate USB timeout."""
 
@@ -56,12 +52,12 @@ class USBSession(Session):
     """
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         """Return list of resources for this type of USB device"""
         raise NotImplementedError
 
     @classmethod
-    def get_low_level_info(cls):
+    def get_low_level_info(cls) -> str:
         try:
             ver = usb.__version__
         except AttributeError:
@@ -75,7 +71,22 @@ def get_low_level_info(cls):
 
         return "via PyUSB (%s). Backend: %s" % (ver, backend)
 
-    def _get_timeout(self, attribute):
+    def after_parsing(self) -> None:
+        self.interface = self._intf_cls(
+            int(self.parsed.manufacturer_id, 0),
+            int(self.parsed.model_code, 0),
+            self.parsed.serial_number,
+        )
+
+        for name in ("SEND_END_EN", "TERMCHAR", "TERMCHAR_EN"):
+            attribute = getattr(constants, "VI_ATTR_" + name)
+            self.attrs[attribute] = attributes.AttributesByID[attribute].default
+
+        # Force setting the timeout to get the proper value
+        attribute = constants.VI_ATTR_TMO_VALUE
+        self.set_attribute(attribute, attributes.AttributesByID[attribute].default)
+
+    def _get_timeout(self, attribute: ResourceAttribute) -> int:
         if self.interface:
             if self.interface.timeout == 2 ** 32 - 1:
                 self.timeout = None
@@ -83,7 +94,7 @@ def _get_timeout(self, attribute):
                 self.timeout = self.interface.timeout / 1000
         return super(USBSession, self)._get_timeout(attribute)
 
-    def _set_timeout(self, attribute, value):
+    def _set_timeout(self, attribute: ResourceAttribute, value: int) -> StatusCode:
         status = super(USBSession, self)._set_timeout(attribute, value)
         timeout = int(self.timeout * 1000) if self.timeout else 2 ** 32 - 1
         timeout = min(timeout, 2 ** 32 - 1)
@@ -91,14 +102,23 @@ def _set_timeout(self, attribute, value):
             self.interface.timeout = timeout
         return status
 
-    def read(self, count):
+    def read(self, count: int) -> Tuple[bytes, StatusCode]:
         """Reads data from device or interface synchronously.
 
         Corresponds to viRead function of the VISA library.
 
-        :param count: Number of bytes to be read.
-        :return: data read, return value of the library call.
-        :rtype: (bytes, VISAStatus)
+        Parameters
+        -----------
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Data read from the device
+        StatusCode
+            Return value of the library call.
+
         """
 
         def _usb_reader():
@@ -110,15 +130,15 @@ def _usb_reader():
                     raise USBTimeoutException()
                 raise
 
-        supress_end_en, _ = self.get_attribute(constants.VI_ATTR_SUPPRESS_END_EN)
+        supress_end_en, _ = self.get_attribute(ResourceAttribute.suppress_end_enabled)
 
         if supress_end_en:
             raise ValueError(
                 "VI_ATTR_SUPPRESS_END_EN == True is currently unsupported by pyvisa-py"
             )
 
-        term_char, _ = self.get_attribute(constants.VI_ATTR_TERMCHAR)
-        term_char_en, _ = self.get_attribute(constants.VI_ATTR_TERMCHAR_EN)
+        term_char, _ = self.get_attribute(ResourceAttribute.termchar)
+        term_char_en, _ = self.get_attribute(ResourceAttribute.termchar_enabled)
 
         return self._read(
             _usb_reader,
@@ -130,18 +150,25 @@ def _usb_reader():
             USBTimeoutException,
         )
 
-    def write(self, data):
+    def write(self, data: bytes) -> Tuple[int, StatusCode]:
         """Writes data to device or interface synchronously.
 
         Corresponds to viWrite function of the VISA library.
 
-        :param data: data to be written.
-        :type data: bytes
-        :return: Number of bytes actually transferred, return value of the library call.
-        :rtype: (int, VISAStatus)
-        """
+        Parameters
+        ----------
+        data : bytes
+            Data to be written.
 
-        send_end, _ = self.get_attribute(constants.VI_ATTR_SEND_END_EN)
+        Returns
+        -------
+        int
+            Number of bytes actually transferred
+        StatusCode
+            Return value of the library call.
+
+        """
+        send_end, _ = self.get_attribute(ResourceAttribute.send_end_enabled)
 
         count = self.interface.write(data)
 
@@ -150,58 +177,60 @@ def write(self, data):
     def close(self):
         self.interface.close()
 
-    def _get_attribute(self, attribute):
+    def _get_attribute(
+        self, attribute: constants.ResourceAttribute
+    ) -> Tuple[Any, StatusCode]:
         """Get the value for a given VISA attribute for this session.
 
         Use to implement custom logic for attributes.
 
-        :param attribute: Resource attribute for which the state query is made
-        :return: The state of the queried attribute for a specified resource, return value of the library call.
-        :rtype: (unicode | str | list | int, VISAStatus)
-        """
+        Parameters
+        ----------
+        attribute : ResourceAttribute
+            Attribute for which the state query is made
 
+        Returns
+        -------
+        Any
+            State of the queried attribute for a specified resource
+        StatusCode
+            Return value of the library call.
+
+        """
         raise UnknownAttribute(attribute)
 
-    def _set_attribute(self, attribute, attribute_state):
+    def _set_attribute(
+        self, attribute: constants.ResourceAttribute, attribute_state: Any
+    ) -> StatusCode:
         """Sets the state of an attribute.
 
         Corresponds to viSetAttribute function of the VISA library.
 
-        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
-        :param attribute_state: The state of the attribute to be set for the specified object.
-        :return: return value of the library call.
-        :rtype: VISAStatus
-        """
-
-        raise UnknownAttribute(attribute)
+        Parameters
+        ----------
+        attribute : constants.ResourceAttribute
+            Attribute for which the state is to be modified. (Attributes.*)
+        attribute_state : Any
+            The state of the attribute to be set for the specified object.
 
-    def after_parsing(self):
-        self.interface = self._intf_cls(
-            int(self.parsed.manufacturer_id, 0),
-            int(self.parsed.model_code, 0),
-            self.parsed.serial_number,
-        )
+        Returns
+        -------
+        StatusCode
+            Return value of the library call.
 
-        for name in ("SEND_END_EN", "TERMCHAR", "TERMCHAR_EN"):
-            attribute = getattr(constants, "VI_ATTR_" + name)
-            self.attrs[attribute] = attributes.AttributesByID[attribute].default
-
-        # Force setting the timeout to get the proper value
-        attribute = constants.VI_ATTR_TMO_VALUE
-        self.set_attribute(attribute, attributes.AttributesByID[attribute].default)
+        """
+        raise UnknownAttribute(attribute)
 
 
 @Session.register(constants.InterfaceType.usb, "INSTR")
 class USBInstrSession(USBSession):
-    """Base class for drivers that communicate with instruments
-    via usb port using pyUSB
-    """
+    """Class for USBTMC devices."""
 
     #: Class to use when instantiating the interface
     _intf_cls = usbtmc.USBTMC
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         out = []
         fmt = (
             "USB%(board)s::%(manufacturer_id)s::%(model_code)s::"
@@ -250,15 +279,13 @@ def list_resources():
 
 @Session.register(constants.InterfaceType.usb, "RAW")
 class USBRawSession(USBSession):
-    """Base class for drivers that communicate with usb raw devices
-    via usb port using pyUSB
-    """
+    """Class for RAW devices."""
 
     #: Class to use when instantiating the interface
     _intf_cls = usbraw.USBRawDevice
 
     @staticmethod
-    def list_resources():
+    def list_resources() -> List[str]:
         out = []
         fmt = (
             "USB%(board)s::%(manufacturer_id)s::%(model_code)s::"
diff --git a/setup.cfg b/setup.cfg
index 606a2ffe..9cbd528f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -70,6 +70,8 @@ per-file-ignores =
     pyvisa-py/protocols/vxi11.py:E221
     pyvisa-py/serial.py:C901
 
+[mypy]
+follow_imports = ignore
 
 [isort]
 multi_line_output = 3
@@ -77,4 +79,5 @@ include_trailing_comma = true
 force_grid_wrap = 0
 use_parentheses = true
 line_length = 88
-known_third_party = pyvisa,numpy,setuptools,typing_extensions
+skip = pyvisa-py/__init__.py
+known_third_party = numpy,setuptools,typing_extensions,pyvisa