Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None

[aeddins-ibm]

Summary

Partial fix for #12765.

That issue points out two problems with BitArray.from_counts / BitArray.from_samples. The more severe problem is that these methods fail with an error if the input (e.g. a counts dictionary) contains only the 0 outcome, and num_bits is left at the default value of None.

The methods try to infer num_bits as the number of bits needed to represent the largest outcome (e.g. bitstring) contained in the input data. If the largest outcome is 0 when converted to an integer (e.g. the bitstring "0"), the number of bits inferred is 0, which can be surprising, and also leads to an error.

This small PR interprets 0 as represented by 1 bit instead of by 0 bits, the suggested correct behavior in #12765.

The less severe problem, not fixed here, is that currently any information about num_bits present in the input bitstring lengths gets ignored. That problem seemed less trivial to me. I put some thoughts here.

[qiskit-bot]

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[coveralls]

Pull Request Test Coverage Report for Build 10102416610

Warning: This coverage report may be inaccurate.

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Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 846 unchanged lines in 45 files lost coverage.
• Overall coverage decreased (-0.2%) to 89.7%

---

Files with Coverage Reduction	New Missed Lines	%
qiskit/qobj/common.py	1	95.83%
qiskit/circuit/library/standard_gates/swap.py	1	98.18%
qiskit/primitives/backend_sampler.py	1	98.86%
qiskit/circuit/duration.py	1	70.27%
qiskit/primitives/base/base_estimator.py	1	97.3%
qiskit/transpiler/passes/layout/sabre_pre_layout.py	1	98.88%
qiskit/transpiler/passes/routing/commuting_2q_gate_routing/commuting_2q_gate_router.py	1	99.12%
qiskit/assembler/disassemble.py	2	98.8%
qiskit/transpiler/passes/scheduling/alap.py	2	96.72%
qiskit/transpiler/passes/scheduling/asap.py	2	96.92%

Totals
Change from base Build 10043438194:	-0.2%
Covered Lines:	66662
Relevant Lines:	74317

---

💛 - Coveralls

[ElePT]

Hi @aeddins-ibm, could you add a short release note documenting the fix?

[ElePT]

This fix looks straightforward, I think we should include it in 1.2 and backport it to 1.1

[ElePT]

https://github.com/Mergifyio backport stable/1.1 stable/1.2

[mergify]

backport stable/0.1.2 stable/1.2

❌ No backport have been created

• Backport to branch stable/0.1.2 failed

GitHub error: Branch not found

• #12820 Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None (backport #12800) has been created for branch stable/1.2

[mergify]

backport stable/1.1 stable/1.2

✅ Backports have been created

• #12821 Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None (backport #12800) has been created for branch stable/1.1
• #12820 Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None (backport #12800) has been created for branch stable/1.2

[t-imamichi]

LGTM