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

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StanFromIreland merged 2 commits into from
Jun 24, 2026
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gh-152060: Fix datetime.fromisoformat() raising AssertionError in pure Python #152061

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gh-152060: Fix datetime.fromisoformat() raising AssertionError in pur…
tonghuaroot Jun 24, 2026
e203639
gh-152060: Address review — simplify error message, drop duplicate te…
tonghuaroot Jun 24, 2026
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3 changes: 2 additions & 1 deletion Lib/_pydatetime.py
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Original file line number Diff line number Diff line change
Expand Up @@ -358,7 +358,8 @@ def _find_isoformat_datetime_separator(dtstr):
def _parse_isoformat_date(dtstr):
# It is assumed that this is an ASCII-only string of lengths 7, 8 or 10,
# see the comment on Modules/_datetimemodule.c:_find_isoformat_datetime_separator
assert len(dtstr) in (7, 8, 10)
if len(dtstr) not in (7, 8, 10):
raise ValueError(f"Invalid isoformat string: {dtstr!r}")

@StanFromIreland StanFromIreland Jun 24, 2026

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{dtstr!r} won't match the C impl, but it doesn’t matter as fromisoformat re-raises anyway.

Suggested change
raise ValueError(f"Invalid isoformat string: {dtstr!r}")
raise ValueError(f"Invalid isoformat string")

@tonghuaroot tonghuaroot Jun 24, 2026

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Done, thanks!

year = int(dtstr[0:4])
has_sep = dtstr[4] == '-'

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2 changes: 2 additions & 0 deletions Lib/test/datetimetester.py
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Expand Up @@ -3757,6 +3757,8 @@ def test_fromisoformat_fails_datetime(self):
'2009-04-19T12:30:45+00:00:90', # Time zone field out from range
'2009-04-19T12:30:45-00:90:00', # Time zone field out from range
'2009-04-19T12:30:45-00:00:90', # Time zone field out from range
'2020-2020', # Ambiguous 9-char date portion
'2020-1234', # Ambiguous 9-char date portion

@StanFromIreland StanFromIreland Jun 24, 2026

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Suggested change
'2020-1234', # Ambiguous 9-char date portion

These cases are identical.

@tonghuaroot tonghuaroot Jun 24, 2026

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Done, thanks!

]

for bad_str in bad_strs:
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst
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@@ -0,0 +1,3 @@
Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
instead of :exc:`ValueError` for some malformed strings in the pure-Python
implementation, matching the C implementation.
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