Ensuring instruction labels are strings when provided to the Instruction constructor. (#7671)

* Fixes #7667 by ensuring that instruction labels are strings when provided to the Instruction constructor.

* Optimizing speed of instruction label type checking.

* Switching to an equality rather than reference check for instruction labels.

* Adding a release note for label type checking on creation.

* Use Sphinx cross-ref in release note

* Fix release note

* Add ctrl_state to non-parameter kwargs

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

qiskit/circuit/instruction.py

@@ -69,6 +69,7 @@ class Instruction:
 
         Raises:
             CircuitError: when the register is not in the correct format.
+            TypeError: when the optional label is provided, but it is not a string.
         """
         if not isinstance(num_qubits, int) or not isinstance(num_clbits, int):
             raise CircuitError("num_qubits and num_clbits must be integer.")
@@ -86,6 +87,8 @@ class Instruction:
         #       already set is a temporary work around that can be removed after
         #       the next stable qiskit-aer release
         if not hasattr(self, "_label"):
+            if label is not None and not isinstance(label, str):
+                raise TypeError("label expects a string or None")
             self._label = label
         # tuple (ClassicalRegister, int), tuple (Clbit, bool) or tuple (Clbit, int)
         # when the instruction has a conditional ("if")

releasenotes/notes/type-check-instruction-labels-on-creation-8399dd8b5d72f272.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    :class:`~.circuit.Instruction` labels are now type-checked on instruction creation.

test/python/circuit/test_extensions_standard.py

@@ -1455,7 +1455,7 @@ class TestStandardMethods(QiskitTestCase):
                 # gate_class is abstract
                 continue
             sig = signature(gate_class)
-            free_params = len(set(sig.parameters) - {"label"})
+            free_params = len(set(sig.parameters) - {"label", "ctrl_state"})
             try:
                 if gate_class == PauliGate:
                     # special case due to PauliGate using string parameters
@@ -1508,7 +1508,7 @@ class TestStandardMethods(QiskitTestCase):
                 # n_qubits argument is no longer supported.
                 free_params = 2
             else:
-                free_params = len(set(sig.parameters) - {"label"})
+                free_params = len(set(sig.parameters) - {"label", "ctrl_state"})
             try:
                 if gate_class == PauliGate:
                     # special case due to PauliGate using string parameters

test/python/circuit/test_instructions.py

@@ -692,6 +692,19 @@ class TestInstructions(QiskitTestCase):
             for instruction in instruction_list:
                 self.assertIs(instruction.condition[0], sentinel_register)
 
+    def test_label_type_enforcement(self):
+        """Test instruction label type enforcement."""
+        with self.subTest("accepts string labels"):
+            instruction = Instruction("h", 1, 0, [], label="label")
+            self.assertEqual(instruction.label, "label")
+        with self.subTest("raises when a non-string label is provided to constructor"):
+            with self.assertRaisesRegex(TypeError, r"label expects a string or None"):
+                Instruction("h", 1, 0, [], label=0)
+        with self.subTest("raises when a non-string label is provided to setter"):
+            with self.assertRaisesRegex(TypeError, r"label expects a string or None"):
+                instruction = HGate()
+                instruction.label = 0
+
 
 if __name__ == "__main__":
     unittest.main()