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I don't have the repo handy to make a patch against 3.5, but would an addition like this do?
in Lib/_strptime.py:
+ elif iso_week != -1 and iso_year == -1:
+ raise ValueError("'%Y' directive is ambiguous in combination with '%V'. Use '%G' instead.")
+ elif julian == -1 and iso_week != -1 and iso_weekday != -1:
+ year, julian = _calc_julian_from_V(iso_year, iso_week, iso_weekday)
In Lib/test/test_strptime.py:
 def test_ValueError(self):
+ # Make sure ValueError is raised when match fails or format is bad
+ self.assertRaises(ValueError, _strptime._strptime, data_string="1905 52",
+ format="%Y %V")