update CCSDS Spacepackets API and docs

README.md

@@ -74,7 +74,8 @@ coverage run -m pytest
 
 # Documentation
 
-The documentation is built with Sphinx
+The documentation is built with Sphinx and new documentation should be written using the
+[NumPy format](https://www.sphinx-doc.org/en/master/usage/extensions/example_numpy.html#example-numpy).
 
 Install the required dependencies first:
 

spacepackets/ccsds/spacepacket.py

@@ -172,8 +172,7 @@ def pack(self) -> bytearray:
 
 
 class SpacePacketHeader(AbstractSpacePacket):
-    """This class encapsulates the space packet header.
-    Packet reference: Blue Book CCSDS 133.0-B-2"""
+    """This class encapsulates the space packet header. Packet reference: Blue Book CCSDS 133.0-B-2"""
 
     def __init__(
         self,
@@ -187,6 +186,10 @@ def __init__(
     ):
         """Create a space packet header with the given field parameters.
 
+        The data length field can also be set from the total packet length by using the
+        :py:meth:`set_data_len_from_packet_len` method after construction of the space packet
+        header object.
+
         >>> sph = SpacePacketHeader(packet_type=PacketType.TC, apid=0x42, seq_count=0, data_len=12)
         >>> hex(sph.apid)
         '0x42'
@@ -201,17 +204,28 @@ def __init__(
         >>> sph.packet_seq_control
         PacketSeqCtrl(seq_flags=<SequenceFlags.UNSEGMENTED: 3>, seq_count=0)
 
-        :param packet_type: 0 for Telemetery, 1 for Telecommands
-        :param apid: Application Process ID, should not be larger
-            than 11 bits, deciaml 2074 or hex 0x7ff
-        :param seq_count: Source sequence counter, should not be larger than 0x3fff or
-            decimal 16383
-        :param data_len: Contains a length count C that equals one fewer than the length of the
-            packet data field. Should not be larger than 65535 bytes
-        :param ccsds_version:
-        :param sec_header_flag: Secondary header flag, or False by default.
-        :param seq_flags:
-        :raises ValueError: On invalid parameters
+        Parameters
+        -----------
+        packet_type: PacketType
+            0 for Telemetery, 1 for Telecommands
+        apid: int
+            Application Process ID, should not be larger than 11 bits, deciaml 2074 or hex 0x7ff
+        seq_count: int
+            Source sequence counter, should not be larger than 0x3fff or decimal 16383
+        data_len: int
+            Contains a length count C that equals one fewer than the length of the packet data
+            field. Should not be larger than 65535 bytes
+        sec_header_flag: bool
+            Secondary header flag, or False by default.
+        seq_flags:
+            Sequence flags, defaults to unsegmented.
+        ccsds_version: int
+            Version of the CCSDS packet. Defaults to 0b000
+
+        Raises
+        --------
+        ValueError:
+            On invalid parameters
         """
         if data_len > pow(2, 16) - 1 or data_len < 0:
             raise ValueError(
@@ -225,6 +239,52 @@ def __init__(
         self._psc = PacketSeqCtrl(seq_flags=seq_flags, seq_count=seq_count)
         self.data_len = data_len
 
+    @classmethod
+    def tc(
+        cls,
+        apid: int,
+        seq_count: int,
+        data_len: int,
+        sec_header_flag: bool = False,
+        seq_flags: SequenceFlags = SequenceFlags.UNSEGMENTED,
+        ccsds_version: int = 0b000,
+    ):
+        """Create a space packet header with the given field parameters for a telecommand packet.
+        Calls the default constructor with the packet type set to telecommand.
+        """
+        cls(
+            packet_type=PacketType.TC,
+            apid=apid,
+            seq_count=seq_count,
+            data_len=data_len,
+            sec_header_flag=sec_header_flag,
+            seq_flags=seq_flags,
+            ccsds_version=ccsds_version,
+        )
+
+    @classmethod
+    def tm(
+        cls,
+        apid: int,
+        seq_count: int,
+        data_len: int,
+        sec_header_flag: bool = False,
+        seq_flags: SequenceFlags = SequenceFlags.UNSEGMENTED,
+        ccsds_version: int = 0b000,
+    ):
+        """Create a space packet header with the given field parameters for a telemetry packet.
+        Calls the default constructor with the packet type set to telemetry.
+        """
+        cls(
+            packet_type=PacketType.TM,
+            apid=apid,
+            seq_count=seq_count,
+            data_len=data_len,
+            sec_header_flag=sec_header_flag,
+            seq_flags=seq_flags,
+            ccsds_version=ccsds_version,
+        )
+
     @classmethod
     def from_composite_fields(
         cls,
@@ -289,6 +349,19 @@ def sec_header_flag(self, value):
     def seq_count(self):
         return self._psc.seq_count
 
+    def set_data_len_from_packet_len(self, packet_len: int):
+        """Sets the data length field from the given total packet length. The total packet length
+        must be at least 7 bytes.
+
+        Raises
+        -------
+        ValueError:
+            The passed packet length is smaller than the minimum expected 7 bytes.
+        """
+        if packet_len < CCSDS_HEADER_LEN + 1:
+            raise ValueError("specified total packet length too short")
+        self.data_len = packet_len - CCSDS_HEADER_LEN - 1
+
     @seq_count.setter
     def seq_count(self, seq_cnt):
         self._psc.seq_count = seq_cnt