Updates to the target methods (#601)

* Change target types from Iterable to Collection

* Proper error message for when no targets are given

pulser-core/pulser/json/abstract_repr/serializer.py

@@ -16,7 +16,7 @@
 
 import inspect
 import json
-from collections.abc import Iterable
+from collections.abc import Collection
 from itertools import chain
 from typing import TYPE_CHECKING, Any, Union, cast
 
@@ -156,16 +156,16 @@ def serialize_abstract_sequence(
             res["variables"][var.name]["value"] = [var.dtype()] * var.size
 
     def unfold_targets(
-        target_ids: QubitId | Iterable[QubitId],
+        target_ids: QubitId | Collection[QubitId],
     ) -> QubitId | list[QubitId]:
         if isinstance(target_ids, (int, str)):
             return target_ids
 
-        targets = list(cast(Iterable, target_ids))
+        targets = list(cast(Collection, target_ids))
         return targets if len(targets) > 1 else targets[0]
 
     def convert_targets(
-        target_ids: Union[QubitId, Iterable[QubitId]],
+        target_ids: Union[QubitId, Collection[QubitId]],
         force_list_out: bool = False,
     ) -> Union[int, list[int]]:
         target_array = np.array(unfold_targets(target_ids))

pulser-core/pulser/sequence/sequence.py

@@ -19,7 +19,7 @@
 import json
 import os
 import warnings
-from collections.abc import Iterable, Mapping
+from collections.abc import Collection, Mapping
 from typing import (
     Any,
     Generic,
@@ -529,7 +529,7 @@ def _set_slm_mask_dmm(self, dmm_id: str, targets: set[QubitId]) -> None:
 
     @seq_decorators.store
     def config_slm_mask(
-        self, qubits: Iterable[QubitId], dmm_id: str = "dmm_0"
+        self, qubits: Collection[QubitId], dmm_id: str = "dmm_0"
     ) -> None:
         """Setup an SLM mask by specifying the qubits it targets.
 
@@ -545,7 +545,7 @@ def config_slm_mask(
         pulse added after this operation.
 
         Args:
-            qubits: Iterable of qubit ID's to mask during the first global
+            qubits: Collection of qubit ID's to mask during the first global
                 pulse of the sequence.
             dmm_id: Id of the DMM channel to use in the device.
         """
@@ -846,7 +846,7 @@ def declare_channel(
         self,
         name: str,
         channel_id: str,
-        initial_target: Optional[Union[QubitId, Iterable[QubitId]]] = None,
+        initial_target: Optional[Union[QubitId, Collection[QubitId]]] = None,
     ) -> None:
         """Declares a new channel to the Sequence.
 
@@ -901,7 +901,7 @@ def declare_channel(
             try:
                 cond = any(
                     isinstance(t, Parametrized)
-                    for t in cast(Iterable, initial_target)
+                    for t in cast(Collection, initial_target)
                 )
             except TypeError:
                 cond = isinstance(initial_target, Parametrized)
@@ -930,7 +930,7 @@ def declare_channel(
             else:
                 # "_target" call is not saved
                 self._target(
-                    cast(Union[Iterable, QubitId], initial_target), name
+                    cast(Union[Collection, QubitId], initial_target), name
                 )
 
         # Manually store the channel declaration as a regular call
@@ -1319,14 +1319,14 @@ def add_dmm_detuning(
     @seq_decorators.store
     def target(
         self,
-        qubits: Union[QubitId, Iterable[QubitId]],
+        qubits: Union[QubitId, Collection[QubitId]],
         channel: str,
     ) -> None:
         """Changes the target qubit of a 'Local' channel.
 
         Args:
             qubits: The new target for this channel. Must correspond to a
-                qubit ID in device or an iterable of qubit IDs, when
+                qubit ID in device or a collection of qubit IDs, when
                 multi-qubit addressing is possible.
             channel: The channel's name provided when declared. Must be
                 a channel with 'Local' addressing.
@@ -1336,14 +1336,14 @@ def target(
     @seq_decorators.store
     def target_index(
         self,
-        qubits: Union[int, Iterable[int], Parametrized],
+        qubits: Union[int, Collection[int], Parametrized],
         channel: str,
     ) -> None:
         """Changes the target qubit of a 'Local' channel.
 
         Args:
             qubits: The new target for this channel. Must correspond to a
-                qubit index or an iterable of qubit indices, when multi-qubit
+                qubit index or an collection of qubit indices, when multi-qubit
                 addressing is possible.
                 A qubit index is a number between 0 and the number of qubits.
                 It is then converted to a Qubit ID using the order in which
@@ -1993,21 +1993,26 @@ def _add(
     @seq_decorators.block_if_measured
     def _target(
         self,
-        qubits: Union[Iterable[QubitId], QubitId, Parametrized],
+        qubits: Union[Collection[QubitId], QubitId, Parametrized],
         channel: str,
         _index: bool = False,
     ) -> None:
         self._validate_channel(channel, block_eom_mode=True)
         channel_obj = self._schedule[channel].channel_obj
         try:
             qubits_set = (
-                set(cast(Iterable, qubits))
+                set(cast(Collection, qubits))
                 if not isinstance(qubits, str)
                 else {qubits}
             )
         except TypeError:
             qubits_set = {qubits}
 
+        if not qubits_set:
+            raise ValueError(
+                "Need at least one qubit to target but none were given."
+            )
+
         if channel_obj.addressing != "Local":
             raise ValueError("Can only choose target of 'Local' channels.")
         elif (

tests/test_sequence.py

@@ -958,6 +958,8 @@ def test_target(reg, device):
         seq.target("q3", "ch1")
     with pytest.raises(ValueError, match="can target at most 1 qubits"):
         seq.target(["q1", "q5"], "ch0")
+    with pytest.raises(ValueError, match="Need at least one qubit to target"):
+        seq.target([], "ch0")
 
     assert seq._schedule["ch0"][-1] == _TimeSlot("target", -1, 0, {"q1"})
     seq.target("q4", "ch0")