Make `MultiQubitFrame.operators' attribute read-only

povm_toolbox/quantum_info/multi_qubit_frame.py

@@ -64,7 +64,28 @@ def __init__(self, list_operators: list[Operator]) -> None:
         self._array: np.ndarray
         self._informationally_complete: bool
 
-        self.operators = list_operators
+        self._num_operators = len(list_operators)
+        self._dimension = list_operators[0].dim[0]
+        for frame_op in list_operators:
+            if not (self._dimension == frame_op.dim[0] and self._dimension == frame_op.dim[1]):
+                raise ValueError(
+                    f"Frame operators need to be square ({frame_op.dim[0]},{frame_op.dim[1]}) and "
+                    "all of the same dimension."
+                )
+
+        self._operators = list_operators
+
+        self._pauli_operators = None
+
+        self._array = np.ndarray((self.dimension**2, self.num_operators), dtype=complex)
+        for k, frame_op in enumerate(list_operators):
+            self._array[:, k] = matrix_to_double_ket(frame_op.data)
+
+        self._informationally_complete = bool(
+            np.linalg.matrix_rank(self._array) == self.dimension**2
+        )
+
+        self._check_validity()
 
     def __repr__(self) -> str:
         """Return the string representation of a :class:`.MultiQubitFrame` instance."""
@@ -91,32 +112,6 @@ def operators(self) -> list[Operator]:
         """Return the list of frame operators."""
         return self._operators
 
-    @operators.setter
-    def operators(self, new_operators: list[Operator]):
-        """Set the frame operators."""
-        self._num_operators = len(new_operators)
-        self._dimension = new_operators[0].dim[0]
-        for frame_op in new_operators:
-            if not (self._dimension == frame_op.dim[0] and self._dimension == frame_op.dim[1]):
-                raise ValueError(
-                    f"Frame operators need to be square ({frame_op.dim[0]},{frame_op.dim[1]}) and "
-                    "all of the same dimension."
-                )
-
-        self._operators = new_operators
-
-        self._pauli_operators = None
-
-        self._array = np.ndarray((self.dimension**2, self.num_operators), dtype=complex)
-        for k, frame_op in enumerate(new_operators):
-            self._array[:, k] = matrix_to_double_ket(frame_op.data)
-
-        self._informationally_complete = bool(
-            np.linalg.matrix_rank(self._array) == self.dimension**2
-        )
-
-        self._check_validity()
-
     @property
     def pauli_operators(self) -> list[dict[str, complex]]:
         """Convert the internal frame operators to Pauli form.

releasenotes/notes/read-only-operators-bae10ffedf6b683c.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    The :attr:`.MultiQubitFrame.operators` attribute of :class:`.MultiQubitFrame` objects is now
+    read-only.

test/quantum_info/test_multi_qubit_povm.py

@@ -38,6 +38,13 @@ def test_invalid_operators(self):
             _ = MultiQubitPOVM(
                 list_operators=[0.9 * Operator.from_label("0"), Operator.from_label("1")]
             )
+        with self.subTest("Non-square operators") and self.assertRaises(ValueError):
+            _ = MultiQubitPOVM(
+                [
+                    Operator(np.array([[1, 0, 0], [0, 0, 0]])),
+                    Operator(np.array([[0, 0, 0], [0, 1, 0]])),
+                ]
+            )
 
     def test_dimension(self):
         """Test dimension attribute"""
@@ -134,15 +141,6 @@ def test_repr(self):
             )
             self.assertEqual(povm.__repr__(), f"MultiQubitPOVM(num_qubits=2)<4> at {hex(id(povm))}")
 
-    def test_operators_setter(self):
-        """Test the ``__repr__`` method."""
-        povm = MultiQubitPOVM([Operator.from_label("0"), Operator.from_label("1")])
-        with self.subTest("Non-square operators") and self.assertRaises(ValueError):
-            povm.operators = [
-                Operator(np.array([[1, 0, 0], [0, 0, 0]])),
-                Operator(np.array([[0, 0, 0], [0, 1, 0]])),
-            ]
-
     def test_pauli_operators(self):
         """Test errors are raised  correctly for the ``pauli_operators`` attribute."""
         povm = MultiQubitPOVM([Operator(np.eye(3))])