Warning on `add_bits` with layout information

[kaldag]

Summary

The method add_bits does not check if the circuit has layout information. The layout information is not updated when the method add_bits is called by a circuit already having layout information. A warning is added to inform if a circuit has layout information.

Details and comments

A UserWarning provided to add_bits in quantumcircuit.py.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 16872034759

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.04%) to 87.692%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/symbol_expr.rs	1	74.74%
crates/qasm2/src/expr.rs	1	93.63%
crates/qasm2/src/lex.rs	1	92.27%
<!--	Total:	3

Totals
Change from base Build 16837695320:	0.04%
Covered Lines:	82389
Relevant Lines:	93953

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💛 - Coveralls

[kaldag]

Thanks for this Kalyan. We should also add a test that the warning is raised, both for directly adding qubits, for adding them implicitly via add_register(QuantumRegister), and also that the warning isn't raised if you only add clbits (which aren't in the layout).

I think the warning could perhaps be clearer about why it's a problem, and we might want to document in add_bits why it's a problem to add qubits to circuit that's been laid out.

Currently in the implementation, we warn but continue, which leaves the circuit in an invalid state. I'm actually thinking now: perhaps this ought to be an exception, not a warning? You need to do QuantumCircuit.layout = None before adding bits, or you're invalidating the coherence?

Added documentation and test cases as suggested.

[kaldag]

Thanks Kalyan - this looks sensible, the review's just minor wording stuff then it's mergeable.

Thanks a lot for the review @jakelishman. I have made all the changes you recommended.