gh-151770: Fix datetime.fromisoformat() assertion on an out-of-range month with a 24:00 time by tonghuaroot · Pull Request #151771 · python/cpython · GitHub

datetime.fromisoformat() reached days_in_month() with month == 0 when the input combined an out-of-range (below 1) month with a 24:00 time, for example "2009-00-01T24:00:00". On a --with-pydebug build this tripped assert(month >= 1) in days_in_month (Modules/_datetimemodule.c); the pure-Python implementation in Lib/_pydatetime.py raised AssertionError on the same input. On a release build both already raised ValueError.

The 24:00 midnight-rollover guard validated only the upper month bound (month <= 12) before the days_in_month() call, and the parser does not range-check the month before that point, so a month below 1 slipped through. The matching upper-bound input "2009-13-01T24:00:00" was already rejected by the month <= 12 check.

This adds the missing month >= 1 lower bound to the guard in both implementations (mirroring the existing check_date_args validation), so an out-of-range month consistently raises ValueError. A regression test is added next to the existing out-of-range-month case in datetimetester.py.

Issue: datetime.fromisoformat() hits a debug-build assertion on an out-of-range month combined with a 24:00 time #151770

…range month with a 24:00 time The 24:00 midnight-rollover path validated only the upper month bound before calling days_in_month(), so a month below 1 reached assert(month >= 1) on a debug build (and AssertionError in the pure Python implementation). Add the missing lower bound to both so an out-of-range month consistently raises ValueError.

Just a few little nits, the core change is correct.

While we're dealing with this assert, can you please update it (on L58) like so:

- assert 1 <= month <= 12, month
+ assert 1 <= month <= 12, f"month must be in 1..12, not {month}"

Suggested change

      
            Fix :meth:`datetime.datetime.fromisoformat` raising an unexpected error on an
          
            out-of-range month combined with a ``24:00`` time, such as
          
            ``"2009-00-01T24:00:00"``. It now consistently raises :exc:`ValueError`.
          
            Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
          
            instead of :exc:`ValueError` for an out-of-range month combined with a
          
            ``24:00`` time.

Suggested change

      
                        "2009-00-01T24:00:00",          # Month out of range (below)
          
                        "2009-00-01T24:00:00",          # Month below range

Thanks @StanFromIreland — applied all three.

LGTM

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.
🐍🍒⛏🤖

GH-151809 is a backport of this pull request to the 3.15 branch.

Sorry, @tonghuaroot and @StanFromIreland, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 1fb874cc076e771c39a7bbc650dce386e3c5b7a0 3.13

GH-151810 is a backport of this pull request to the 3.14 branch.

@tonghuaroot are you able to do the backport (see bot comment above)?

…onth w/ a 24:00 time (GH-151771) (#151810) (cherry picked from commit 1fb874c) Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com> Co-authored-by: Stan Ulbrych <stan@python.org>

…onth w/ a 24:00 time (GH-151771) (#151809) (cherry picked from commit 1fb874c) Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com> Co-authored-by: Stan Ulbrych <stan@python.org>

Thanks @StanFromIreland! I looked into the 3.13 backport and I don't think it's applicable there.

On 3.13, datetime.fromisoformat() doesn't support a 24:00 time at all — it rejects hour 24 as out of range before any month handling:

>>> datetime.fromisoformat('9999-12-31T24:00:00')   # 3.13
ValueError: hour must be in 0..23

The 24:00 → next-day normalization that this fix touches (the became_next_day path) was only added in 3.14 via gh-102450 / #105856, which merged after 3.13 had already branched, so the out-of-range-month-with-24:00 case can't occur on 3.13 — the out-of-range month is already rejected cleanly there:

>>> datetime.fromisoformat('2009-13-01T24:00:00')   # 3.13
ValueError: month must be in 1..12

That's why the cherry-pick conflicts: the whole code path the patch modifies isn't present on 3.13. I think the needs backport to 3.13 label can be dropped. Happy to do the backport if I've missed something.

Thanks for checking!

…nth w/ a 24:00 time (python#151771) Co-authored-by: Stan Ulbrych <stan@python.org>

tonghuaroot requested review from StanFromIreland and pganssle as code owners June 20, 2026 07:21

bedevere-app Bot mentioned this pull request Jun 20, 2026

datetime.fromisoformat() hits a debug-build assertion on an out-of-range month combined with a 24:00 time #151770

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bedevere-app Bot added the awaiting review label Jun 20, 2026

StanFromIreland reviewed Jun 20, 2026

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Address review: align assert message, reword NEWS, tighten test comment

01091e4

StanFromIreland reviewed Jun 20, 2026

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Comment thread Modules/_datetimemodule.c Outdated

Remove additional parentheses

0e24642

StanFromIreland approved these changes Jun 20, 2026

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bedevere-app Bot added awaiting merge and removed awaiting review labels Jun 20, 2026

StanFromIreland added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes needs backport to 3.15 pre-release feature fixes, bugs and security fixes labels Jun 20, 2026

StanFromIreland merged commit 1fb874c into python:main Jun 20, 2026
108 of 110 checks passed

bedevere-app Bot removed the awaiting merge label Jun 20, 2026

bedevere-app Bot removed the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label Jun 20, 2026

miss-islington-app Bot assigned StanFromIreland Jun 20, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Jun 20, 2026

StanFromIreland removed the needs backport to 3.13 bugs and security fixes label Jun 21, 2026

This was referenced Jun 24, 2026

datetime.fromisoformat() raises AssertionError instead of ValueError in the pure-Python implementation #152060

Closed

gh-152060: Fix datetime.fromisoformat() raising AssertionError in pure Python #152061

Merged

pyctrl pushed a commit to pyctrl/cpython that referenced this pull request Jul 5, 2026

pythongh-151770 : Fix datetime.fromisoformat() on an out-of-range mo…

a871679

…nth w/ a 24:00 time (python#151771) Co-authored-by: Stan Ulbrych <stan@python.org>

                           "2009-04-01T12:30:90",          # Second out of range
                           "2009-04-01T12:90:45",          # Minute out of range
                           "2009-04-01T25:30:45",          # Hour out of range
+                          "2009-00-01T24:00:00",          # Month out of range (below)

	"2009-00-01T24:00:00", # Month out of range (below)
	"2009-00-01T24:00:00", # Month below range

gh-151770: Fix datetime.fromisoformat() assertion on an out-of-range month with a 24:00 time by tonghuaroot · Pull Request #151771 · python/cpython · GitHub

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gh-151770: Fix datetime.fromisoformat() assertion on an out-of-range month with a 24:00 time - #151771

gh-151770: Fix datetime.fromisoformat() assertion on an out-of-range month with a 24:00 time#151771StanFromIreland merged 3 commits intopython:mainpython/cpython:mainfrom tonghuaroot:gh-151770-fromiso-month-24htonghuaroot/cpython:gh-151770-fromiso-month-24hCopy head branch name to clipboard

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gh-151770: Fix datetime.fromisoformat() assertion on an out-of-range month with a 24:00 time#151771
StanFromIreland merged 3 commits into
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tonghuaroot:gh-151770-fromiso-month-24h

tonghuaroot commented Jun 20, 2026 •
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