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gh-124549: Warn on conflicting directives in datetime.*.strptime #132524

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gh-124549: Warn on conflicting directives in datetime.*.strptime #132524

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eb63f10
Raise a warning when passing conflicting directives
StanFromIreland Apr 14, 2025
e670d33
Reword
StanFromIreland May 19, 2025
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9 changes: 9 additions & 0 deletions Doc/library/datetime.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -2765,6 +2765,15 @@ Notes:
: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`.

(11)
When parsing a string using :meth:`~.datetime.strptime`, if more than one
directive of the same time unit is used, a SyntaxWarning is raised as
only the last directive gets parsed and applied to the datetime object. For
example, if the format string contains both ``%Y`` and ``%y``, like ``%Y%y``,
only the last directive of the string (``%y``) gets used on the resulting
datetime object.


.. rubric:: Footnotes

.. [#] If, that is, we ignore the effects of Relativity
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7 changes: 5 additions & 2 deletions Lib/_strptime.py
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Expand Up @@ -357,12 +357,15 @@ def pattern(self, format):
day_of_month_in_format = False
def repl(m):
format_char = m[1]
nonlocal year_in_format, day_of_month_in_format
match format_char:
case 'Y' | 'y' | 'G':
nonlocal year_in_format
if year_in_format:
import warnings
warnings.warn("When using conflicting directives only the last one will be used.",
SyntaxWarning, skip_file_prefixes=(os.path.dirname(__file__),))
year_in_format = True
case 'd':
nonlocal day_of_month_in_format
day_of_month_in_format = True
return self[format_char]
format = re_sub(r'%([OE]?\\?.?)', repl, format)
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2025-04-14-18-18-00.gh-issue-124549.hcsdgf.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Raise a warning in :func:`datetime.datetime.strptime` and :func:`datetime.date.strptime`
when conflicting directives are provided.
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