gh-152079: Fix C datetime.fromisoformat() dropping sub-second UTC offset#152087
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StanFromIreland merged 2 commits intoJun 25, 2026
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…TC offset The C accelerator's tzinfo_from_isoformat_results() collapsed any offset with a zero whole-second part to timezone.utc, discarding the parsed sub-second microseconds. So '+00:00:00.000001' round-tripped through isoformat()/fromisoformat() lost its 1-microsecond offset, while pure Python preserved it. Only short-circuit to UTC when both the whole-second and sub-second parts are zero.
StanFromIreland
approved these changes
Jun 24, 2026
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Thanks @tonghuaroot for the PR, and @StanFromIreland for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15. |
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GH-152174 is a backport of this pull request to the 3.15 branch. |
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GH-152175 is a backport of this pull request to the 3.14 branch. |
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GH-152176 is a backport of this pull request to the 3.13 branch. |
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Merged, thanks. |
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The C accelerator's
tzinfo_from_isoformat_results()collapsed any UTC offset with a zerowhole-second part to
timezone.utc, discarding the parsed sub-second microseconds. As aresult
'2020-06-15T12:34:56+00:00:00.000001'round-tripped throughisoformat()/fromisoformat()lost its 1-microsecond offset, while the pure-Python implementationpreserved it.
This short-circuits to
timezone.utconly when both the whole-second and sub-second partsare zero. A plain
+00:00offset still returnstimezone.utc; the non-zero sub-secondcase now falls through to the existing
new_timezone(new_delta(...))path and ispreserved, matching pure Python.
I verified against a full C-vs-pure-Python differential corpus that the only inputs whose
behaviour changes are exactly these zero-whole-second sub-second offsets (75 previously
divergent cases now agree) and that no new divergence is introduced.
A round-trip regression test is added; it runs under both the C (
_Fast) and pure-Python(
_Pure) test classes.Fixes #152079. Sibling to #152060, a separate
fromisoformat()defect in the pure-Pythonimplementation.