Discard cache for standard gates in assign_parameters (#13557)

* Fix: Discard cache when assigning new parameters in `CircuitData`.

* Docs: Add release note

* Test: Add test case

* Docs: Add relevant comment

crates/circuit/src/circuit_data.rs

@@ -1392,15 +1392,12 @@ impl CircuitData {
                             #[cfg(feature = "cache_pygates")]
                             {
                                 // Standard gates can all rebuild their definitions, so if the
-                                // cached py_op exists, update the `params` attribute and clear out
-                                // any existing cache.
-                                if let Some(borrowed) = previous.py_op.get() {
-                                    borrowed
-                                        .bind(py)
-                                        .getattr(params_attr)?
-                                        .set_item(parameter, new_param)?;
-                                    borrowed.bind(py).setattr("_definition", py.None())?
-                                }
+                                // cached py_op exists, discard it to prompt the instruction
+                                // to rebuild its cached python gate upon request later on. This is
+                                // done to avoid an unintentional duplicated reference to the same gate
+                                // instance in python. For more information, see
+                                // https://github.com/Qiskit/qiskit/issues/13504
+                                previous.py_op.take();
                             }
                         } else {
                             // Track user operations we've seen so we can rebind their definitions.

releasenotes/notes/fix-assign-parameters-ffa284ebde429704.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fix incorrect behavior in :class:`.CircuitData` in which, upon parameter assignment,
+    we attempted to modify the cached operation inside of a ``PackedInstruction``. Now
+    we instead discard said cache prompting the ``PackedInstruction`` to build a new Python
+    operation should it be needed.

test/python/transpiler/test_basis_translator.py

@@ -21,6 +21,7 @@
 from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
 from qiskit import transpile
 from qiskit.circuit import Gate, Parameter, EquivalenceLibrary, Qubit, Clbit, Measure
+from qiskit.circuit.equivalence_library import StandardEquivalenceLibrary as std_eq_lib
 from qiskit.circuit.classical import expr, types
 from qiskit.circuit.library import (
     HGate,
@@ -481,6 +482,22 @@ def test_different_bits(self):
         out = BasisTranslator(std_eqlib, basis).run(circuit_to_dag(base))
         self.assertEqual(set(out.count_ops(recurse=True)), basis)
 
+    def test_correct_parameter_assignment(self):
+        """Test correct parameter assignment from an equivalence during translation"""
+        rx_key = next(key for key in std_eq_lib.keys() if key.name == "rx")
+
+        # The circuit doesn't need to be parametric.
+        qc = QuantumCircuit(1)
+        qc.rx(0.5, 0)
+
+        BasisTranslator(
+            equivalence_library=std_eq_lib,
+            target_basis=["cx", "id", "rz", "sx", "x"],
+        )(qc)
+
+        inst = std_eq_lib._get_equivalences(rx_key)[0].circuit.data[0]
+        self.assertEqual(inst.params, inst.operation.params)
+
 
 class TestUnrollerCompatability(QiskitTestCase):
     """Tests backward compatability with the Unroller pass.