Improve docs and tests of observables_restricted_to_subsystem

Qiskit-Extensions · May 8, 2024 · 911187a · 911187a
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diff --git a/circuit_knitting/utils/observable_grouping.py b/circuit_knitting/utils/observable_grouping.py
@@ -45,14 +45,16 @@ def observables_restricted_to_subsystem(
     A :class:`~qiskit.quantum_info.PauliList` will be returned if a :class:`~qiskit.quantum_info.PauliList` is provided; otherwise,
     a ``list[Pauli]`` will be returned.
 
+    Any phase information will be discarded, consistent with the standard behavior when slicing a Pauli.
+
     Args:
         qubits: The qubits in a subsystem
         global_observables: The list of observables
 
     Returns:
         Each :class:`~qiskit.quantum_info.Pauli` restricted to the subsystem.
 
-    >>> observables_restricted_to_subsystem([1, 3], PauliList(["IXYZ", "ZZXX"]))
+    >>> observables_restricted_to_subsystem([1, 3], PauliList(["IXYZ", "iZZXX"]))
     PauliList(['IY', 'ZX'])
 
     """

diff --git a/test/utils/test_observable_grouping.py b/test/utils/test_observable_grouping.py
@@ -77,6 +77,11 @@ def test_most_general_observable(self):
             PauliList(["XX", "YY", "ZZ"]),
             PauliList.from_symplectic(np.zeros((3, 0)), np.zeros((3, 0))),
         ),
+        (
+            (2, 1),
+            [Pauli("XYZ"), Pauli("iZYX"), Pauli("-ZXZ")],
+            [Pauli("YX"), Pauli("YZ"), Pauli("XZ")],
+        ),
     )
     @unpack
     def test_observables_restricted_to_subsystem(self, qubits, observables, expected):