bpo-36959: Fix checks for invalid ISO date formats in _strptime

4e29c31f40859fe444b81586135374976482c309
commit 4e29c31f40859fe444b81586135374976482c309
@@ -358,8 +358,6 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
     tz = -1
     gmtoff = None
     gmtoff_fraction = 0
-    # Default to -1 to signify that values not known; not critical to have,
-    # though
     iso_week = week_of_year = None
     week_of_year_start = None
     # weekday and julian defaulted to None so as to signal need to calculate
@@ -483,26 +481,28 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
                     else:
                         tz = value
                         break
-    # Deal with the cases where ambiguities arize
+
+    # Deal with the cases where ambiguities arise
     # don't assume default values for ISO week/year
-    if year is None and iso_year is not None:
-        if iso_week is None or weekday is None:
-            raise ValueError("ISO year directive '%G' must be used with "
-                             "the ISO week directive '%V' and a weekday "
-                             "directive ('%A', '%a', '%w', or '%u').")
-        if julian is not None:
-            raise ValueError("Day of the year directive '%j' is not "
-                             "compatible with ISO year directive '%G'. "
-                             "Use '%Y' instead.")
-    elif week_of_year is None and iso_week is not None:
-        if weekday is None:
+    if iso_year is None and iso_week is not None:
+        if year is None or weekday is None:
             raise ValueError("ISO week directive '%V' must be used with "
                              "the ISO year directive '%G' and a weekday "
                              "directive ('%A', '%a', '%w', or '%u').")
         else:
             raise ValueError("ISO week directive '%V' is incompatible with "
                              "the year directive '%Y'. Use the ISO year '%G' "
                              "instead.")
+    elif iso_year is not None:
+        if julian is not None:
+            raise ValueError("Day of the year directive '%j' is not "
+                             "compatible with ISO year directive '%G'. "
+                             "Use '%Y' instead.")
+        elif iso_week is None or weekday is None:
+            raise ValueError("ISO year directive '%G' must be used with "
+                             "the ISO week directive '%V' and a weekday "
+                             "directive ('%A', '%a', '%w', or '%u').")
+
 
     leap_year_fix = False
     if year is None:

@@ -219,25 +219,49 @@ def test_ValueError(self):
             else:
                 self.fail("'%s' did not raise ValueError" % bad_format)
 
-        # Ambiguous or incomplete cases using ISO year/week/weekday directives
-        # 1. ISO week (%V) is specified, but the year is specified with %Y
-        # instead of %G
-        with self.assertRaises(ValueError):
-            _strptime._strptime("1999 50", "%Y %V")
-        # 2. ISO year (%G) and ISO week (%V) are specified, but weekday is not
-        with self.assertRaises(ValueError):
-            _strptime._strptime("1999 51", "%G %V")
-        # 3. ISO year (%G) and weekday are specified, but ISO week (%V) is not
-        for w in ('A', 'a', 'w', 'u'):
-            with self.assertRaises(ValueError):
-                _strptime._strptime("1999 51","%G %{}".format(w))
-        # 4. ISO year is specified alone (e.g. time.strptime('2015', '%G'))
-        with self.assertRaises(ValueError):
-            _strptime._strptime("2015", "%G")
-        # 5. Julian/ordinal day (%j) is specified with %G, but not %Y
-        with self.assertRaises(ValueError):
-            _strptime._strptime("1999 256", "%G %j")
+        msg_week_no_year_or_weekday = r"ISO week directive '%V' must be used with " \
+            r"the ISO year directive '%G' and a weekday directive " \
+            r"\('%A', '%a', '%w', or '%u'\)."
+        msg_week_not_compatible = r"ISO week directive '%V' is incompatible with " \
+            r"the year directive '%Y'. Use the ISO year '%G' instead."
+        msg_julian_not_compatible = r"Day of the year directive '%j' is not " \
+            r"compatible with ISO year directive '%G'. Use '%Y' instead."
+        msg_year_no_week_or_weekday = r"ISO year directive '%G' must be used with " \
+            r"the ISO week directive '%V' and a weekday directive " \
+            r"\('%A', '%a', '%w', or '%u'\)."
+
+        locale_time = _strptime.LocaleTime()
 
+        # Ambiguous or incomplete cases using ISO year/week/weekday directives
+        subtests = [
+            # 1. ISO week (%V) is specified, but the year is specified with %Y
+            # instead of %G
+            ("1999 50", "%Y %V", msg_week_no_year_or_weekday),
+            ("1999 50 5", "%Y %V %u", msg_week_not_compatible),
+            # 2. ISO year (%G) and ISO week (%V) are specified, but weekday is not
+            ("1999 51", "%G %V", msg_year_no_week_or_weekday),
+            # 3. ISO year (%G) and weekday are specified, but ISO week (%V) is not
+            ("1999 {}".format(locale_time.f_weekday[5]), "%G %A",
+                msg_year_no_week_or_weekday),
+            ("1999 {}".format(locale_time.a_weekday[5]), "%G %a",
+                msg_year_no_week_or_weekday),
+            ("1999 5", "%G %w", msg_year_no_week_or_weekday),
+            ("1999 5", "%G %u", msg_year_no_week_or_weekday),
+            # 4. ISO year is specified alone (e.g. time.strptime('2015', '%G'))
+            ("2015", "%G", msg_year_no_week_or_weekday),
+            # 5. Julian/ordinal day (%j) is specified with %G, but not %Y
+            ("1999 256", "%G %j", msg_julian_not_compatible),
+            ("1999 50 5 256", "%G %V %u %j", msg_julian_not_compatible),
+            # ISO week specified alone
+            ("50", "%V", msg_week_no_year_or_weekday),
+            # ISO year is unspecified, falling back to year
+            ("50 5", "%V %u", msg_week_no_year_or_weekday),
+        ]
+
+        for (data_string, format, message) in subtests:
+            with self.subTest(data_string=data_string, format=format):
+                with self.assertRaisesRegex(ValueError, message):
+                    _strptime._strptime(data_string, format)
 
     def test_strptime_exception_context(self):
         # check that this doesn't chain exceptions needlessly (see #17572)

diff --git a/Misc/NEWS.d/next/Library/2019-05-18-15-50-14.bpo-36959.ew6WZ4.rst b/Misc/NEWS.d/next/Library/2019-05-18-15-50-14.bpo-36959.ew6WZ4.rst
@@ -0,0 +1,5 @@
+Fix checks for invalid combinations of ISO format strings in strptime.
+
+(This will change ValueError messages for certain invalid combinations.)
+
+Patch by Gordon P. Hemsley.