Invalidate parameters cache on circuit copy (backport #12619) (#12620)

* 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`.

(cherry picked from commit b6c6166)

# Conflicts:
#	qiskit/circuit/quantumcircuit.py
#	test/python/circuit/test_parameters.py

* Fix conflict

* Fix conflict in test

* Update test/python/circuit/test_parameters.py

---------

Co-authored-by: Jake Lishman <[email protected]>
Co-authored-by: Elena Peña Tapia <[email protected]>

qiskit/circuit/quantumcircuit.py

@@ -2147,6 +2147,8 @@ def copy_empty_like(self, name: str | None = None) -> "QuantumCircuit":
 
         cpy._calibrations = copy.deepcopy(self._calibrations)
         cpy._metadata = copy.deepcopy(self._metadata)
+        # Invalidate parameters caching.
+        cpy._parameters = None
 
         if name:
             cpy.name = name

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

@@ -176,6 +176,29 @@ def test_get_parameters_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_bind_parameters_anonymously(self):
         """Test setting parameters by insertion order anonymously"""
         phase = Parameter("phase")