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")