Improve compose() errors for circuit width mismatches#15945
Improve compose() errors for circuit width mismatches#15945TSS99 wants to merge 3 commits intoQiskit:mainfrom
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qiskit/circuit/quantumcircuit.py
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| if other.num_qubits > dest.num_qubits: | ||
| raise CircuitError( | ||
| "Trying to compose with another QuantumCircuit which has more 'in' edges." | ||
| f"Cannot compose a circuit with {other.num_qubits} qubit(s) " |
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The point here is that other cannot have more qubits/clbits than dest. Can we make the message reflect that? E.g.
Cannot compose onto a circuit that has less qubits ({other.num_qubits} > {dest.num_qubits})
for all of them?
| bfs_successors as core_bfs_successors, descendants as core_descendants, | ||
| }; | ||
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| fn dag_compose_width_error(bit_kind: &str, other: usize, dest: usize) -> String { |
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We should tell cargo to inline this since it's just a call to format!
| fn dag_compose_width_error(bit_kind: &str, other: usize, dest: usize) -> String { | |
| #[inline] | |
| fn dag_compose_width_error(bit_kind: &str, other: usize, dest: usize) -> String { |
The even nicer solution would be to have Rust-based DAGCircuitErrors and have an error variant for this with the correctly formatted string. But given that the error type is imported from Python we can't do that yet.
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Summary
Fixes #13935.
This updates
QuantumCircuit.compose()to raise clearer error messages when the circuit being composed is wider than the destination circuit.Specifically:
What changed
"more 'in' edges"error with user-facingCircuitErrormessagesWhy
The previous error message exposed internal graph terminology that is hard for new users to interpret. This change makes the failure mode more actionable and easier to understand.
Testing
AI disclosure
I used OpenAI Codex (GPT-5) for implementation assistance, and I reviewed and understood all changes before submitting this PR.