Invalidate parameters cache on circuit copy (#12619)

Previously, the caching of the parameter view could persist between
copies of the circuit because it was part of the `copy.copy`.

qiskit/circuit/quantumcircuit.py

@@ -3674,6 +3674,8 @@ def copy_empty_like(
         cpy._data = CircuitData(
             self._data.qubits, self._data.clbits, global_phase=self._data.global_phase
         )
+        # Invalidate parameters caching.
+        cpy._parameters = None
 
         cpy._calibrations = _copy.deepcopy(self._calibrations)
         cpy._metadata = _copy.deepcopy(self._metadata)

releasenotes/notes/fix-parameter-cache-05eac2f24477ccb8.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    The :attr:`.QuantumCircuit.parameters` attribute will now correctly be empty
+    when using :meth:`.QuantumCircuit.copy_empty_like` on a parametric circuit.
+    Previously, an internal cache would be copied over without invalidation.
+    Fix `#12617 <https://github.com/Qiskit/qiskit/issues/12617>`__.

test/python/circuit/test_parameters.py

@@ -203,6 +203,29 @@ def test_parameters_property_by_index(self):
         for i, vi in enumerate(v):
             self.assertEqual(vi, qc.parameters[i])
 
+    def test_parameters_property_independent_after_copy(self):
+        """Test that any `parameters` property caching is invalidated after a copy operation."""
+        a = Parameter("a")
+        b = Parameter("b")
+        c = Parameter("c")
+
+        qc1 = QuantumCircuit(1)
+        qc1.rz(a, 0)
+        self.assertEqual(set(qc1.parameters), {a})
+
+        qc2 = qc1.copy_empty_like()
+        self.assertEqual(set(qc2.parameters), set())
+
+        qc3 = qc1.copy()
+        self.assertEqual(set(qc3.parameters), {a})
+        qc3.rz(b, 0)
+        self.assertEqual(set(qc3.parameters), {a, b})
+        self.assertEqual(set(qc1.parameters), {a})
+
+        qc1.rz(c, 0)
+        self.assertEqual(set(qc1.parameters), {a, c})
+        self.assertEqual(set(qc3.parameters), {a, b})
+
     def test_get_parameter(self):
         """Test the `get_parameter` method."""
         x = Parameter("x")