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Make a shallow copy of metadata in QuantumCircuit <> DAG conversion #16142

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Make a shallow copy of metadata in QuantumCircuit <> DAG conversion #16142

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8 changes: 7 additions & 1 deletion crates/circuit/src/converters.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,13 @@ impl<'a, 'py> FromPyObject<'a, 'py> for QuantumCircuitData<'py> {
Ok(QuantumCircuitData {
data: data_borrowed.borrow().inner.clone(),
name: ob.getattr(intern!(py, "name"))?.extract()?,
metadata: ob.getattr(intern!(py, "metadata")).ok(),
metadata: ob.getattr(intern!(py, "metadata")).ok().and_then(|m| {
if m.is_none() {
None
} else {
m.call_method0(intern!(py, "copy")).ok()
}
}),
transpile_layout: ob.getattr(intern!(py, "layout")).ok(),
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@jakelishman jakelishman May 5, 2026

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This place (Rust-space extraction of the fields from a Python-space object) is most likely not the right place to put this - that's going to cause the copy in the wrong places, I think. It so happens that circuit_to_dag calls this right now (I think?), but that's just one of the users.

})
}
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2 changes: 1 addition & 1 deletion qiskit/converters/dag_to_circuit.py
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Expand Up @@ -68,7 +68,7 @@ def dag_to_circuit(dag, copy_operations=True):
name=name,
global_phase=dag.global_phase,
)
circuit.metadata = dag.metadata or {}
circuit.metadata = dag.metadata.copy() if dag.metadata else {}
circuit._data = circuit_data
circuit._duration = dag._duration
circuit._unit = dag._unit
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13 changes: 13 additions & 0 deletions test/python/converters/test_circuit_to_dag.py
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Expand Up @@ -207,6 +207,19 @@ def test_wire_order_failures(self):
with self.assertRaisesRegex(ValueError, "does not contain exactly the same"):
circuit_to_dag(qc, clbit_order=cr[[0, 1, 1]])

def test_circuit_to_dag_metadata_is_copied(self):
"""circuit_to_dag should return a DAG with a copy of the metadata, not a reference."""
qc = QuantumCircuit(1, metadata={"key": "original"})
dag = circuit_to_dag(qc)
self.assertIsNot(dag.metadata, qc.metadata)
self.assertEqual(dag.metadata, qc.metadata)

def test_dag_to_circuit_metadata_is_copied(self):
"""dag_to_circuit should return a circuit with a copy of the metadata, not a reference."""
qc = QuantumCircuit(1, metadata={"key": "original"})
result = dag_to_circuit(circuit_to_dag(qc))
self.assertIsNot(result.metadata, qc.metadata)


if __name__ == "__main__":
unittest.main(verbosity=2)