Reject bad values in SparsePauliOp.paulis setter (backport #10437)

qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

@@ -235,6 +235,14 @@ def paulis(self):
     def paulis(self, value):
         if not isinstance(value, PauliList):
             value = PauliList(value)
+        if value.num_qubits != self.num_qubits:
+            raise ValueError(
+                f"incorrect number of qubits: expected {self.num_qubits}, got {value.num_qubits}"
+            )
+        if len(value) != len(self.paulis):
+            raise ValueError(
+                f"incorrect number of operators: expected {len(self.paulis)}, got {len(value)}"
+            )
         self._pauli_list = value
 
     @property

releasenotes/notes/sparse-pauli-op-constraint-pauli-setter-52f6f89627d1937c.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The setter for :attr:`.SparsePauliOp.paulis` will now correctly reject attempts to set the
+    attribute with incorrectly shaped data, rather than silently allowing an invalid object to be
+    created.  See `#10384 <https://github.com/Qiskit/qiskit-terra/issues/10384>`__.

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

@@ -1015,6 +1015,14 @@ def test_assign_parameters(self):
         with self.subTest(msg="fully bound"):
             self.assertTrue(np.allclose(bound.coeffs.astype(complex), [1, 3, 6]))
 
+    def test_paulis_setter_rejects_bad_inputs(self):
+        """Test that the setter for `paulis` rejects different-sized inputs."""
+        op = SparsePauliOp(["XY", "ZX"], coeffs=[1, 1j])
+        with self.assertRaisesRegex(ValueError, "incorrect number of qubits"):
+            op.paulis = PauliList([Pauli("X"), Pauli("Y")])
+        with self.assertRaisesRegex(ValueError, "incorrect number of operators"):
+            op.paulis = PauliList([Pauli("XY"), Pauli("ZX"), Pauli("YZ")])
+
 
 if __name__ == "__main__":
     unittest.main()