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[3.12] gh-70647: update docs to mention the datetime 1900 year default 2/29 issue (#131534)

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

Every four years people encounter this because it just isn't obvious.
This moves the footnote up to a note with a code example.

We'd love to change the default year value for datetime but doing
that could have other consequences for existing code.  This documented
workaround *always* works.

* doctest code within note is bad, dedent.
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Gregory P. Smith 2025-03-20 21:50:10 -07:00 • committed by GitHub
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Doc/library/datetime.rst
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@ -2526,7 +2526,24 @@ Broadly speaking, ``d.strftime(fmt)`` acts like the :mod:`time` module's
For the :meth:`.datetime.strptime` class method, the default value is
``1900-01-01T00:00:00.000``: any components not specified in the format string
will be pulled from the default value. [#]_
will be pulled from the default value.
.. note::
When used to parse partial dates lacking a year, :meth:`~.datetime.strptime`
will raise when encountering February 29 because its default year of 1900 is
*not* a leap year. Always add a default leap year to partial date strings
before parsing.
.. doctest::
>>> from datetime import datetime
>>> value = "2/29"
>>> datetime.strptime(value, "%m/%d")
Traceback (most recent call last):
...
ValueError: day is out of range for month
>>> datetime.strptime(f"1904 {value}", "%Y %m/%d")
datetime.datetime(1904, 2, 29, 0, 0)
Using ``datetime.strptime(date_string, format)`` is equivalent to::
@ -2652,6 +2669,11 @@ Notes:
for formats ``%d``, ``%m``, ``%H``, ``%I``, ``%M``, ``%S``, ``%j``, ``%U``,
``%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.
.. rubric:: Footnotes
.. [#] If, that is, we ignore the effects of Relativity
@ -2665,5 +2687,3 @@ 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.