From 9b7a6286327b0e03d258316dde7419ca0a7e10f3 Mon Sep 17 00:00:00 2001 From: "Gregory P. Smith" <68491+gpshead@users.noreply.github.com> Date: Sat, 20 Dec 2025 23:03:24 -0800 Subject: [PATCH] [3.13] gh-70647: Better promote how to safely parse yearless dates in datetime. (GH-116179) (#143038) * gh-70647: Better promote how to safely parse yearless dates in datetime. (GH-116179) * 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. * Update to match the error message. * remove no longer referenced footnote * ignore the warning in the doctest * use Petr's suggestion for the docs to hide the warning processing * cover date.strptime (3.14) as well * remove date.strptime mentions from 3.14 --- Doc/library/datetime.rst | 39 +++++++++++++++++++++++++++++++++++---- 1 file changed, 35 insertions(+), 4 deletions(-) diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst index 99a979038adb..64356c9b6b04 100644 --- a/Doc/library/datetime.rst +++ b/Doc/library/datetime.rst @@ -2589,7 +2589,40 @@ 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 the default year of 1900 is + *not* a leap year. Always add a default leap year to partial date strings + before parsing. + + +.. testsetup:: + + # doctest seems to turn the warning into an error which makes it + # show up and require matching and prevents the actual interesting + # exception from being raised. + # Manually apply the catch_warnings context manager + import warnings + catch_warnings = warnings.catch_warnings() + catch_warnings.__enter__() + warnings.simplefilter("ignore") + +.. testcleanup:: + + catch_warnings.__exit__() + +.. doctest:: + + >>> from datetime import datetime + >>> value = "2/29" + >>> datetime.strptime(value, "%m/%d") + Traceback (most recent call last): + ... + ValueError: day 29 must be in range 1..28 for month 2 in year 1900 + >>> datetime.strptime(f"1904 {value}", "%Y %m/%d") + datetime.datetime(1904, 2, 29, 0, 0) Using ``datetime.strptime(date_string, format)`` is equivalent to:: @@ -2720,7 +2753,7 @@ Notes: 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 is not a leap year. Users run into this bug every four years... + parser (1900) is not a leap year. Users run into that bug every leap year. .. doctest:: @@ -2747,5 +2780,3 @@ Notes: .. [#] See R. H. van Gent's `guide to the mathematics of the ISO 8601 calendar `_ for a good explanation. - -.. [#] Passing ``datetime.strptime('Feb 29', '%b %d')`` will fail since 1900 is not a leap year. -- 2.47.3