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GH-70647: Deprecate strptime day of month parsing without a year present to avoid leap-year bugs by gpshead · Pull Request #117107 · python/cpython · GitHub
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GH-70647: Deprecate strptime day of month parsing without a year present to avoid leap-year bugs #117107

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Merged
encukou merged 9 commits into from
Apr 3, 2024
Merged

GH-70647: Deprecate strptime day of month parsing without a year present to avoid leap-year bugs #117107

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4b929e4
Deprecate strptime leap-year ambiguous MM-DD parsing.
gpshead Mar 20, 2024
b0eacf9
Fix the stacklevel and remove example from the warning message.
gpshead Mar 20, 2024
5851b0b
Simplify _assertNotWarns error message, removing a typo.
gpshead Mar 20, 2024
a068dac
add a unittest for _assertNotWarns.
gpshead Mar 20, 2024
333ba57
Document the behavior deprecation.
gpshead Mar 29, 2024
b12e7e5
minor doc cleanup.
gpshead Apr 1, 2024
d4c994e
adjust example code comments.
gpshead Apr 1, 2024
92e30b8
Fix doctest
encukou Apr 2, 2024
f1bd314
Use skip_file_prefixes for the warning
encukou Apr 3, 2024
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Document the behavior deprecation.
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@gpshead
gpshead committed Mar 29, 2024
commit 333ba5789d638650cf4ec41c47c29ce22d43aaa5
33 changes: 33 additions & 0 deletions Doc/library/datetime.rst
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Expand Up @@ -1079,6 +1079,24 @@ Other constructors, all class methods:
time tuple. See also :ref:`strftime-strptime-behavior` and
:meth:`datetime.fromisoformat`.

.. versionchanged:: 3.13

If *format* specifies a day of month without a year a
:exc:`DeprecationWarning` is now emitted. This is to avoid a quadrennial
leap year bug in code seeking to parse only a month and day as the
default year used in absence of one in the format is not a leap year.
Such *format* values may raise an error as of Python 3.15. The
workaround is to always include a year in your *format*. If parsing
*date_string* values that do not have a year, explicitly add a year that
is a leap year before parsing:

.. doctest::

>>> from datetime import datetime
>>> date_string = "02/29"
>>> when = datetime.strptime(f"{date_string};1984", "%m/%d;%Y")
>>> when.strftime("%B %d") # doctest: +SKIP
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pganssle marked this conversation as resolved.
'February 29'


Class attributes:
Expand Down Expand Up @@ -2657,6 +2675,21 @@ Notes:
for formats ``%d``, ``%m``, ``%H``, ``%I``, ``%M``, ``%S``, ``%j``, ``%U``,
``%W``, and ``%V``. Format ``%y`` does require a leading zero.

(10)
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,
just append an explicit dummy leap year. Otherwise your code will raise an
exception when it encounters leap day as the default year used by the parser
is not a leap year. Users run into this bug every four years...

>>> strptime(f"{month_day};1984", "%m/%d;%Y") # problem mitigated!

.. 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 leapyear. See :gh:`70647`.

.. rubric:: Footnotes

.. [#] If, that is, we ignore the effects of Relativity
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