ENH: to_datetime support iso week year (16607)

pandas/_libs/tslib.pyx

@@ -3827,25 +3827,30 @@ def array_strptime(ndarray[object] values, object fmt,
             if iso_week == -1 or weekday == -1:
                 raise ValueError("ISO year directive '%G' must be used with "
                                  "the ISO week directive '%V' and a weekday "
-                                 "directive ('%w', or '%u').")
+                                 "directive ('%A', '%a', '%w', or '%u').")
             if julian != -1:
                 raise ValueError("Day of the year directive '%j' is not "
                                  "compatible with ISO year directive '%G'. "
                                  "Use '%Y' instead.")
-        elif week_of_year == -1 and iso_week != -1:
+        elif year != -1 and week_of_year == -1 and iso_week != -1:
             if weekday == -1:
                 raise ValueError("ISO week directive '%V' must be used with "
                                  "the ISO year directive '%G' and a weekday "
-                                 "directive ('%w', or '%u').")
+                                 "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.")
 
         # If we know the wk of the year and what day of that wk, we can figure
         # out the Julian day of the year.
-        if julian == -1 and week_of_year != -1 and weekday != -1:
-            week_starts_Mon = True if week_of_year_start == 0 else False
-            julian = _calc_julian_from_U_or_W(year, week_of_year, weekday,
-                                                week_starts_Mon)
-        elif iso_year != -1 and iso_week != -1 and weekday != -1:
-            year, julian = _calc_julian_from_V(iso_year, iso_week, weekday + 1)
+        if julian == -1 and weekday != -1:
+            if week_of_year != -1:
+                week_starts_Mon = True if week_of_year_start == 0 else False
+                julian = _calc_julian_from_U_or_W(year, week_of_year, weekday,
+                                                    week_starts_Mon)
+            elif iso_year != -1 and iso_week != -1:
+                year, julian = _calc_julian_from_V(iso_year, iso_week, weekday + 1)
 
         # Cannot pre-calculate datetime_date() since can change in Julian
         # calculation and thus could have different value for the day of the wk

pandas/tests/indexes/datetimes/test_tools.py

@@ -177,12 +177,40 @@ class TestToDatetime(object):
 
     def test_to_datetime_iso_week_year_format(self):
         data = [
-            ['2015-53-1', '%G-%V-%u',
-            datetime(2015, 12, 28, 0, 0)]
+            ['2015-1-1', '%G-%V-%u',
+            datetime(2014, 12, 29, 0, 0)], #negative ordinal (date in previous year)
+            ['2015-1-4', '%G-%V-%u',
+            datetime(2015, 1, 1, 0, 0)],
+            ['2015-1-7', '%G-%V-%u',
+            datetime(2015, 1, 4, 0, 0)] 
             ]
         for s, format, dt in data:
             assert to_datetime(s, format=format) == dt
 
+    def test_ValueError_iso_week_year(self):
+        # 1. ISO week (%V) is specified, but the year is specified with %Y
+        # instead of %G
+        with pytest.raises(ValueError):
+           to_datetime("1999 50", format="%Y %V")
+        # 2. ISO year (%G) and ISO week (%V) are specified, but weekday is not
+        with pytest.raises(ValueError):
+            to_datetime("1999 51", format="%G %V")
+        # 3. ISO year (%G) and weekday are specified, but ISO week (%V) is not
+        for w in ('A', 'a', 'w', 'u'):
+            with pytest.raises(ValueError):
+                to_datetime("1999 51", format="%G %{}".format(w))
+        # 4. ISO year is specified alone 
+        with pytest.raises(ValueError):
+            to_datetime("2015", format="%G")
+        # 5. Julian/ordinal day (%j) is specified with %G, but not %Y
+        with pytest.raises(ValueError):
+            to_datetime("1999 256", format="%G %j")
+        #6. ISO week (%V) and weekday are specified, but ISO year (%G) is not 
+        for w in ('A', 'a', 'w', 'u'):
+            with pytest.raises(ValueError):
+                to_datetime("51 11", format="%V %{}".format(w))
+
+
     def test_to_datetime_dt64s(self):
         in_bound_dts = [
             np.datetime64('2000-01-01'),