Fix parameter handling for NLocal(..., flatten=True) and standard gates

crates/circuit/src/circuit_data.rs

@@ -1361,12 +1361,9 @@ impl CircuitData {
                             let Param::ParameterExpression(expr) = &params[parameter] else {
                                 return Err(inconsistent());
                             };
-                            params[parameter] = match bind_expr(
-                                expr.bind_borrowed(py),
-                                &param_ob,
-                                value.as_ref(),
-                                true,
-                            )? {
+                            let new_param =
+                                bind_expr(expr.bind_borrowed(py), &param_ob, value.as_ref(), true)?;
+                            params[parameter] = match new_param.clone_ref(py) {
                                 Param::Obj(obj) => {
                                     return Err(CircuitError::new_err(format!(
                                         "bad type after binding for gate '{}': '{}'",
@@ -1382,8 +1379,13 @@ impl CircuitData {
                             #[cfg(feature = "cache_pygates")]
                             {
                                 // Standard gates can all rebuild their definitions, so if the
-                                // cached py_op exists, just clear out any existing cache.
+                                // 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())?
                                 }
                             }

releasenotes/notes/fix-cached-params-update-4d2814b698fa76b4.yaml

@@ -0,0 +1,17 @@
+---
+fixes:
+  - |
+    Fixed a bug in :meth:`.QuantumCircuit.assign_parameters`, occurring when assigning parameters
+    to standard gates whose definition has already been triggered. In this case, the new values
+    were not properly propagated to the gate instances. While the circuit itself was still
+    compiled as expected, inspecting the individual operations would still show the old parameter.
+
+    For example::
+
+      from qiskit.circuit.library import EfficientSU2
+
+      circuit = EfficientSU2(2, flatten=True)
+      circuit.assign_parameters([1.25] * circuit.num_parameters, inplace=True)
+      print(circuit.data[0].operation.params)  # would print θ[0] instead of 1.25
+
+    Fixed `#13478 <https://github.com/Qiskit/qiskit/issues/13478>`__.

test/python/circuit/library/test_nlocal.py

@@ -488,6 +488,21 @@ def test_initial_state_as_circuit_object(self):
 
         self.assertCircuitEqual(ref, expected)
 
+    def test_inplace_assignment_with_cache(self):
+        """Test parameters are correctly re-bound in the cached gates.
+
+        This test requires building with the Rust feature "cache_pygates" enabled, otherwise
+        it does not test what it is supposed to.
+
+        Regression test of #13478.
+        """
+        qc = EfficientSU2(2, flatten=True)
+        binds = [1.25] * qc.num_parameters
+
+        qc.assign_parameters(binds, inplace=True)
+        bound_op = qc.data[0].operation
+        self.assertAlmostEqual(bound_op.params[0], binds[0])
+
 
 @ddt
 class TestNLocalFunction(QiskitTestCase):

test/python/circuit/test_parameters.py

@@ -361,6 +361,19 @@ def test_assign_parameters_by_iterable(self):
         self.assertEqual(qc.assign_parameters(dict(zip(qc.parameters, binds)).values()), expected)
         self.assertEqual(qc.assign_parameters(bind for bind in binds), expected)
 
+    def test_assign_parameters_with_cache(self):
+        """Test assigning parameters on a circuit with already triggered cache."""
+        x = Parameter("x")
+        qc = QuantumCircuit(1)
+        qc.append(RZGate(x), [0])  # add via ``append`` to create a CircuitInstruction
+
+        _ = qc.data[0].operation.definition  # trigger building the cache
+
+        binds = [1.2]
+        qc.assign_parameters(binds, inplace=True)
+
+        self.assertAlmostEqual(binds[0], qc.data[0].operation.params[0])
+
     def test_bind_parameters_custom_definition_global_phase(self):
         """Test that a custom gate with a parametrized `global_phase` is assigned correctly."""
         x = Parameter("x")