Raise a warning when passing conflicting directives

Co-authored-by: Mariatta <Mariatta@users.noreply.github.com>

Author: StanFromIreland

Doc/library/datetime.rst

@@ -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
+   directives of the same time unit were 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 on the string (``%y``) gets used on the resulting
+   datetime object.
+
+
 .. rubric:: Footnotes
 
 .. [#] If, that is, we ignore the effects of Relativity

Lib/_strptime.py

@@ -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)

Misc/NEWS.d/next/Library/2025-04-14-18-18-00.gh-issue-124549.hcsdgf.rst

@@ -0,0 +1,2 @@
+Raise a warning in :func:`datetime.datetime.strptime` and :func:`datetime.date.strptime`
+when conflicting directives are provided.