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gh-70647: Better promote how to safely parse yearless dates in datetime. #116179

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Merged
gpshead merged 10 commits into from
Dec 21, 2025
Merged

gh-70647: Better promote how to safely parse yearless dates in datetime. #116179

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d87b20a
gh-70647: Better promote how to safely parse yearless dates in datetime.
gpshead Mar 1, 2024
11f01a3
doctest code within note is bad, dedent.
gpshead Mar 1, 2024
1f07bc6
Merge branch 'main' into docs/datetime/02-29
gpshead Mar 21, 2025
4860a00
Update to match the error message.
gpshead Mar 21, 2025
9c0d4f8
remove no longer referenced footnote
gpshead Mar 21, 2025
8c6fd9b
ignore the warning in the doctest
gpshead Mar 21, 2025
630b32c
Merge branch 'main' into docs/datetime/02-29
gpshead Jun 15, 2025
020ac63
Merge branch 'main' into docs/datetime/02-29
gpshead Dec 21, 2025
6330e73
use Petr's suggestion for the docs to hide the warning processing
gpshead Dec 21, 2025
e29e51a
cover date.strptime (3.14) as well
gpshead Dec 21, 2025
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Merge branch 'main' into docs/datetime/02-29
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@gpshead
gpshead committed Mar 21, 2025
commit 1f07bc60c43cf3a09af95c32da4734d370e88d6b
22 changes: 19 additions & 3 deletions Doc/library/datetime.rst
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Expand Up @@ -2764,9 +2764,23 @@ Notes:
``%W``, and ``%V``. Format ``%y`` does require a leading zero.

(10)
Parsing dates without a year using :meth:`~.datetime.strptime` will fail on
representations of February 29 as that date does not exist in the default
year of 1900.
When parsing a month and day using :meth:`~.datetime.strptime`, always
include a year in the format. If the value you need to parse lacks a year,
append an explicit dummy leap year. Otherwise your code will raise an
exception when it encounters leap day because the default year used by the
parser (1900) is not a leap year. Users run into that bug every leap year.

.. doctest::

>>> month_day = "02/29"
>>> datetime.strptime(f"{month_day};1984", "%m/%d;%Y") # No leap year bug.
datetime.datetime(1984, 2, 29, 0, 0)

.. deprecated-removed:: 3.13 3.15
:meth:`~.datetime.strptime` calls using a format string containing
a day of month without a year now emit a
:exc:`DeprecationWarning`. In 3.15 or later we may change this into
an error or change the default year to a leap year. See :gh:`70647`.

.. rubric:: Footnotes

Expand All @@ -2781,3 +2795,5 @@ Notes:
.. [#] See R. H. van Gent's `guide to the mathematics of the ISO 8601 calendar
<https://web.archive.org/web/20220531051136/https://webspace.science.uu.nl/~gent0113/calendar/isocalendar.htm>`_
for a good explanation.

.. [#] Passing ``datetime.strptime('Feb 29', '%b %d')`` will fail since 1900 is not a leap year.
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