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

gpshead · web-flow · commit b8d3fddba6e9 · 2025-12-20T22:47:40.000-08:00
* 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
diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
@@ -2651,9 +2651,42 @@ Broadly speaking, ``d.strftime(fmt)`` acts like the :mod:`time` module's
 ``time.strftime(fmt, d.timetuple())`` although not all objects support a
 :meth:`~date.timetuple` method.
 
-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. [#]_
+For the :meth:`.datetime.strptime` and :meth:`.date.strptime` class methods,
+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.
+
+.. note::
+   When used to parse partial dates lacking a year, :meth:`.datetime.strptime`
+   and :meth:`.date.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::
 
@@ -2790,7 +2823,7 @@ 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::
 
@@ -2817,5 +2850,3 @@ 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.
