Fix issue with BlueprintCircuit.

crates/accelerate/src/quantum_circuit/intern_context.rs

@@ -95,6 +95,23 @@ impl InternContext {
             slot_lookup: HashMap::new(),
         }
     }
+
+    pub fn __str__(&self, py: Python<'_>) -> PyObject {
+        let mut str = String::new();
+        str.push_str("InternContext\n");
+        str.push_str("  slots:\n");
+        for (idx, s) in self.slots.iter().enumerate() {
+            if let Some(occupied) = s {
+                str.push_str(&*format!(
+                    "    Slot(idx={idx}, use_count={:?}, operands={:?})\n",
+                    occupied.use_count, occupied.operands
+                ));
+            }
+        }
+        str.push_str("  free_slots:\n");
+        str.push_str(&*format!("    {:?}", self.free_slots));
+        str.into_py(py)
+    }
 }
 
 impl Default for InternContext {

qiskit/circuit/library/blueprintcircuit.py

@@ -65,7 +65,7 @@ def _build(self) -> None:
 
     def _invalidate(self) -> None:
         """Invalidate the current circuit build."""
-        self._data = []
+        self._data = super()._new_data()
         self._parameter_table = ParameterTable()
         self.global_phase = 0
         self._is_built = False

qiskit/circuit/quantumcircuit.py

@@ -240,8 +240,13 @@ def __init__(
             "qiskit.circuit.controlflow.builder.ControlFlowBuilderBlock"
         ] = []
 
-        self.qregs: list[QuantumRegister] = []
-        self.cregs: list[ClassicalRegister] = []
+        # Skip initialization if a subclass (e.g. BlueprintCircuit) is overriding
+        # these via properties.
+        if not hasattr(type(self), "qregs"):
+            self.qregs: list[QuantumRegister] = []
+        if not hasattr(type(self), "cregs"):
+            self.cregs: list[ClassicalRegister] = []
+
         self._qubits: list[Qubit] = []
         self._clbits: list[Clbit] = []