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Fix deepcopy for new-style Bit #10411

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merged 4 commits into from
Jul 11, 2023
Merged

Fix deepcopy for new-style Bit #10411

merged 4 commits into from
Jul 11, 2023

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kevinhartman
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@kevinhartman kevinhartman commented Jul 10, 2023 •
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Summary

Overrides the default behavior of copy and deepcopy for Bit to return self for new-style bits.

Details and comments

We choose this behavior because Bits are immutable, and it allows us to deep-copy new-style bits while maintaining equality between the original and the copy. For old-style bits, we leave the behavior unchanged, since some existing code appears to depend on _register getting deep-copied.

Note that we now return self when shallow-copying old-style bits, which we can do safely since Bits themselves are immutable.

Resolves #10409.

@kevinhartman kevinhartman added stable backport potential The bug might be minimal and/or import enough to be port to stable Changelog: Bugfix Include in the "Fixed" section of the changelog labels Jul 10, 2023
@kevinhartman kevinhartman added this to the 0.24.2 milestone Jul 10, 2023
@kevinhartman kevinhartman requested a review from a team as a code owner July 10, 2023 15:42
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jakelishman
jakelishman reviewed Jul 10, 2023
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This looks sensible to me, thanks. I think it would be good to insert a test, if there's a sensible one we can do.

releasenotes/notes/fix-bit-copy-4b2f7349683f616a.yaml Show resolved Hide resolved
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coveralls commented Jul 10, 2023 •
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Pull Request Test Coverage Report for Build 5513114945

  • 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
  • 159 unchanged lines in 8 files lost coverage.
  • Overall coverage increased (+0.008%) to 85.994%
Files with Coverage Reduction New Missed Lines %
qiskit/qasm/init.py 1 80.0%
crates/qasm2/src/lex.rs 2 91.39%
qiskit/extensions/quantum_initializer/squ.py 2 80.0%
qiskit/qasm/pygments/init.py 3 0%
qiskit/qpy/binary_io/circuits.py 11 92.95%
qiskit/qasm/pygments/lexer.py 14 0%
qiskit/tools/jupyter/library.py 43 0%
qiskit/circuit/quantumcircuit.py 83 94.05%
Totals Coverage Status
Change from base Build 5490783039: 0.008%
Covered Lines: 71629
Relevant Lines: 83295
💛 - Coveralls

@kevinhartman kevinhartman mentioned this pull request Jul 10, 2023
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kevinhartman commented Jul 10, 2023 •
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I've updated the description since the PR now only changes the behavior of deepcopy for new-style bits.

@kevinhartman kevinhartman changed the title Make Bit copy return self. Fix deepcopy for new-style Bit Jul 10, 2023
jakelishman
jakelishman approved these changes Jul 11, 2023
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Looks good, thanks for the spotting the problem and solving it!

qiskit/circuit/bit.py
Comment on lines +133 to +140
# Old-style bits need special handling for now, since some code seems
# to rely on their registers getting deep-copied.
bit = type(self).__new__(type(self))
bit._register = copy.deepcopy(self._register, memo)
bit._index = self._index
bit._hash = self._hash
bit._repr = self._repr
return bit
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Yeah, in fairness, we were wrong before when we didn't recurse the deepcopy onto the register. It likely would have caused a weird inconsistency where the reg[0]._register is not reg, which is an invariant of old-style registers/bits at the moment. I can't immediately see why that has knock-on effects, but I guess it shouldn't be surprising.

@jakelishman jakelishman added this pull request to the merge queue Jul 11, 2023
Merged via the queue into Qiskit:main with commit e9c86a5 Jul 11, 2023
mergify bot pushed a commit that referenced this pull request Jul 11, 2023
@kevinhartman @mergify
Fix deepcopy for new-style Bit (#10411)
66f0fcc 
* Make bit copy return self.

* Add release note.

* Preserve existing deepcopy behavior for old bits.

* Add tests to validate copy behaviors.

(cherry picked from commit e9c86a5)
@mergify mergify bot mentioned this pull request Jul 11, 2023
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github-merge-queue bot pushed a commit that referenced this pull request Jul 11, 2023
@mergify @kevinhartman
Fix deepcopy for new-style Bit (#10411) (#10413)
76d52cb 
* Make bit copy return self.

* Add release note.

* Preserve existing deepcopy behavior for old bits.

* Add tests to validate copy behaviors.

(cherry picked from commit e9c86a5)

Co-authored-by: Kevin Hartman <kevin@hart.mn>
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Changelog: Bugfix Include in the "Fixed" section of the changelog stable backport potential The bug might be minimal and/or import enough to be port to stable
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Deepcopy of control flow with standalone clbit results in invalid circuit
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