If input paulis have phase, mutate and warn

qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

@@ -19,6 +19,7 @@
 from collections.abc import Mapping, Sequence, Iterable
 from numbers import Number
 from copy import deepcopy
+import warnings
 
 import numpy as np
 import rustworkx as rx
@@ -256,8 +257,15 @@ def paulis(self, value):
             raise ValueError(
                 f"incorrect number of operators: expected {len(self.paulis)}, got {len(value)}"
             )
-        self.coeffs *= (-1j) ** value.phase
-        self._pauli_list = PauliList.from_symplectic(value.z, value.x, phase=0)
+        if np.any(value.phase):
+            warnings.warn(
+                "Assigning SparsePauliOp.paulis to be a PauliList with nonzero phase sets the phase "
+                "of the given PauliList to zero, absorbing the phase into SparsePauliOp.coeffs.",
+                UserWarning,
+            )
+            self.coeffs *= (-1j) ** value.phase
+            value.phase = 0
+        self._pauli_list = value
 
     @property
     def coeffs(self):

test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py

@@ -1102,13 +1102,15 @@ def test_paulis_setter_absorbs_phase(self):
         coeffs_init = np.array([1, 1j])
         op = SparsePauliOp(["XY", "ZX"], coeffs=coeffs_init)
         paulis_new = PauliList(["-1jXY", "1jZX"])
-        op.paulis = paulis_new
+        with self.assertWarns(UserWarning):
+            # Raise a warning that the RHS is mutated:
+            op.paulis = paulis_new
         # Paulis attribute should have no phase:
         self.assertEqual(op.paulis, PauliList(["XY", "ZX"]))
         # Coeffs attribute should now include that phase:
         self.assertTrue(np.allclose(op.coeffs, coeffs_init * np.array([-1j, 1j])))
-        # Do not mutate the phase of the input array:
-        self.assertTrue(np.allclose(paulis_new.phase, np.array([1, 3])))
+        # The phase of the input array is now zero:
+        self.assertTrue(np.allclose(paulis_new.phase, np.array([0, 0])))
 
     def test_apply_layout_with_transpile(self):
         """Test the apply_layout method with a transpiler layout."""