gh-137165: Standardize non-zero-padded date formatting for `datetime.strftime` by F0RRZZ · Pull Request #139088 · python/cpython

F0RRZZ · 2025-09-18T00:42:03Z

Previously, using non-zero-padded directives (e.g., %-d) in strftime caused test failures on Windows and Android (x86_64), while other platforms worked as expected.
This has been fixed by manually handling such specifiers on these platforms.
Corresponding tests have been added to strftime tests.

On Windows using non-zero-padded directives raised ValueError.

On Android (x86_64) dicrectives were not replaced:

Closes Enhancement Proposal: Standardized Support for Non-Zero-Padded Day and Month Formatting in datetime.strftime #137165

…ftime

mhsmith · 2025-09-18T12:31:55Z

!buildbot iOS

mhsmith · 2025-09-18T12:34:36Z

The implementation looks reasonable, but this should also be covered in the documentation, along with the fact that the - flag apparently has no effect on %y on Apple platforms.

F0RRZZ · 2025-09-18T14:39:29Z

@mhsmith I adapted the tests for iOS and added documentation in the strftime notes.

F0RRZZ · 2025-09-18T16:44:26Z

@StanFromIreland Made all changes

F0RRZZ · 2025-09-19T05:45:40Z

@mhsmith All done, but TSan seems to be stuck

mhsmith

@pganssle: I think this is ready for final review.

F0RRZZ · 2025-11-05T07:34:49Z

@pganssle Just a gentle ping on this PR review🙏

github-actions · 2026-04-29T12:31:00Z

This PR is stale because it has been open for 30 days with no activity.

F0RRZZ · 2026-07-28T09:36:10Z

@StanFromIreland gentle ping
should be good for another look whenever you have a sec

ambv · 2026-10-20T22:50:30Z

This PR is superseded by #145018, a newer open pull request for the same issue (gh-137165), and it no longer merges cleanly. Closing to cull the number of open pull requests. Do not reopen this pull request without getting a go-ahead from a triager or core developer.

F0RRZZ · 2026-10-21T01:20:19Z

unified the behavior for unsupported specifiers like on Linux.

I'd rather leave that to separate PR.

@StanFromIreland Since we decided to split this task into two PRs, could you please reopen this one?

read-the-docs-community · 2026-10-21T06:55:55Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #35366220 | 📁 Comparing 1022b42 against main (b577a54)

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Co-authored-by: Muse Code (Meta Muse Spark)

On Windows and Android, every `%-X` was emulated by formatting `%X` and stripping leading zeros. That misbehaved for anything outside the documented numeric codes: non-ASCII was narrowed to a byte (`%-Ť` formatted the day, `%-é` raised OverflowError), `%-%` stuffed an unescaped `%` into the format (`%-%d` gave the day), and `%-z`/`%-Z` bypassed datetime's tzinfo handling. Emulate only `%-d`, `%-m`, `%-H`, `%-I`, `%-M`, `%-S`, `%-j`, `%-U`, `%-W`, `%-V` and `%-y`, and leave everything else to the platform as before. Gate the pure-Python branch the same way, so it no longer consumes `%-%` as a unit on other platforms, which made `%-%f` differ from the C implementation. Use the cached time.strftime instead of importing it on every call. Co-authored-by: Muse Code (Meta Muse Spark)

StanFromIreland · 2026-10-21T17:32:30Z

@ambv, this will require some careful review before merging, and Buildbots will be handy too.

ambv · 2026-10-21T17:37:13Z

@StanFromIreland agreed! My changes are effect of my review. I'll get somebody at the sprint to look at it before we land it.

bedevere-bot · 2026-10-21T18:52:20Z

🤖 New build scheduled with the buildbot fleet by @ambv for commit f74ad49 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F139088%2Fmerge

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Emulate documented numeric dash flags on OpenBSD and account for NetBSD's native zero padding of the short year. Cover 12-hour formatting at midnight, noon and single-digit hours, document the version introducing support, and include datetime.time.strftime in the NEWS entry. Co-authored-by: Muse Code (Meta Muse Spark)

gpshead

@ambv asked me to run this through Claude as a font-of-other-platforms-knowledge reviewer looking for defects. findings:

Test gating by platform.system() is wrong for some Apple platforms (Lib/test/datetimetester.py:1603). platform.system() returns 'iPadOS' on iPads, and tvOS/watchOS/visionOS have their own names too. Those fall into the else branch expecting y:5, where Apple libc gives y:05. DragonFly (FreeBSD-derived libc) likely has the same problem, though I didn't check it. support.is_apple or sys.platform.startswith(('freebsd', 'netbsd')) would be safer and would match the sys.platform gating in _pydatetime.py. platform conditionals are a mess and often need abstracting (what do we have in test.support for these? -gps)
Hardcoded platform list instead of a probe. The Windows/Android/OpenBSD list is duplicated in C (#if) and Python (sys.platform). The new tests assert %-j %-U %-W %-V, %-H %-M %-S and %-I unconditionally, so any other libc without the - flag fails them. AIX and Solaris are the candidates; I did not verify either. A one-time probe in the style of _need_normalize_century() (does time.strftime('%-d', …) give '1'?) would remove the list.
NEWS overstates what the change does. It says "standardized … across all platforms", but the docs and tests in the same PR carve out %-y staying zero-padded on Apple, FreeBSD and NetBSD. Either emulate %-y there too, or soften the NEWS wording.
My own human review comment added below is half addressed (datetimetester.py:1602). The docs describe the %-y inconsistency, but the test just says # See gh-137165 above the platform branch. It needs a comment there saying why expectations differ.
Doc placement. The new paragraph about strftime is appended to note (9), which is the strptime leading-zero note. It reads oddly there and probably deserves its own note, or at least a clear separation.
Edge case: %-%-d. After an un-emulated %-, the scanner does not consume the next character as the conversion. So %-%-d becomes %-2 under emulation, where glibc natively gives %-d. This is the same existing quirk the new test deliberately pins for %-%f, so it is low priority.
Nit: import platform is appended after import warnings, out of alphabetical order; it goes away if finding 1 is fixed with sys.platform.

Address review findings from gpshead: - Emulate the documented numeric dash flags unconditionally in both C and pure-Python implementations, instead of gating by platform. This removes the hardcoded Windows/Android/OpenBSD list (and its duplication), fixes %-y on Apple, FreeBSD and NetBSD where the native libc ignores the flag, and makes the NEWS claim of "across all platforms" accurate. No probe is needed since emulation via %X + lstrip('0') matches native output where the flag already works. - Tests no longer branch by platform.system(), which misclassified iPadOS, tvOS, watchOS, visionOS and DragonFly. Expectations are now unconditional, with an explanatory comment for %-y. Windows %# checks use sys.platform, and the out-of-order `import platform` is gone. - Docs: clarify the strftime paragraph in note (9) with an explicit "For formatting..." transition (kept under the same note number per mhsmith to avoid table clutter), and document that %- is emulated where the platform lacks it. The %-%-d edge case is left as is: it shares the existing %-%f quirk deliberately pinned by test_strftime_dash_flag_passthrough. Co-authored-by: Muse Code (Meta Muse Spark)

marat added 2 commits September 17, 2025 02:04

pythongh-137165: Add non-zero-padded Windows support for datetime.str…

2f6c0ce

…ftime

Refactor make_dash_replacement func and add negative test

8d16901

F0RRZZ requested review from abalkin and pganssle as code owners September 18, 2025 00:42

bedevere-app Bot added the awaiting review label Sep 18, 2025

bedevere-app Bot mentioned this pull request Sep 18, 2025

Enhancement Proposal: Standardized Support for Non-Zero-Padded Day and Month Formatting in datetime.strftime #137165

Open

F0RRZZ changed the title ~~gh-137165: Add non-zero-padded Windows support for datetime.strftime~~ gh-137165: Standardized non-zero-padded date formatting for datetime.strftime Sep 18, 2025

mhsmith mentioned this pull request Sep 18, 2025

gh-137165: Add non-zero-padded Windows support for datetime.strftime #139002

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marat added 2 commits September 18, 2025 16:42

Fix tests for IOS and add docs

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Update docs and news entry

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F0RRZZ force-pushed the issue-137165 branch from 67ec54d to eb5c883 Compare September 18, 2025 14:10

F0RRZZ requested a review from mhsmith September 18, 2025 14:39

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marat added 2 commits September 18, 2025 18:45

Fix 0 seconds/minutes strip and update docs

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Update news entry

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F0RRZZ requested a review from StanFromIreland September 18, 2025 16:44

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marat added 2 commits September 19, 2025 07:46

Unify strftime/ptime notes and refactor code

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Update notes numbers

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Merge branch 'main' into issue-137165

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F0RRZZ requested a review from StanFromIreland February 5, 2026 06:25

F0RRZZ mentioned this pull request Feb 19, 2026

gh-137165: Unify datetime.strftime behavior for unsupported non-zero-padded specifiers #145018

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F0RRZZ force-pushed the issue-137165 branch from 2270997 to 2373670 Compare February 19, 2026 20:02

github-actions Bot added the stale Stale PR or inactive for long period of time. label Apr 29, 2026

github-actions Bot removed the stale Stale PR or inactive for long period of time. label Jul 30, 2026

ambv closed this Oct 20, 2026

StanFromIreland reopened this Oct 21, 2026

bedevere-app Bot added the type-feature A feature request or enhancement label Oct 21, 2026

ambv added 2 commits October 21, 2026 10:21

Merge branch 'main' into issue-137165

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ambv added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Oct 21, 2026

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ambv changed the title ~~gh-137165: Standardized non-zero-padded date formatting for datetime.strftime~~ gh-137165: Standardize non-zero-padded date formatting for datetime.strftime Oct 23, 2026

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