Fix tzawareness_compat for DatetimeIndex comparisons with NaT (#19276)

Author: jbrockmendel

doc/source/whatsnew/v0.23.0.txt

@@ -430,7 +430,7 @@ Conversion
 -
 - Bug in ``.astype()`` to non-ns timedelta units would hold the incorrect dtype (:issue:`19176`, :issue:`19223`, :issue:`12425`)
 - Bug in subtracting :class:`Series` from ``NaT`` incorrectly returning ``NaT`` (:issue:`19158`)
-
+- Bug in comparison of timezone-aware :class:`DatetimeIndex` against ``NaT`` incorrectly raising ``TypeError`` (:issue:`19276`)
 
 Indexing
 ^^^^^^^^

pandas/core/indexes/datetimes.py

@@ -665,7 +665,10 @@ def _assert_tzawareness_compat(self, other):
         if is_datetime64tz_dtype(other):
             # Get tzinfo from Series dtype
             other_tz = other.dtype.tz
-        if self.tz is None:
+        if other is libts.NaT:
+            # pd.NaT quacks both aware and naive
+            pass
+        elif self.tz is None:
             if other_tz is not None:
                 raise TypeError('Cannot compare tz-naive and tz-aware '
                                 'datetime-like objects.')

pandas/tests/indexes/datetimes/test_datetime.py

@@ -286,6 +286,21 @@ def test_comparison_tzawareness_compat(self, op):
         with pytest.raises(TypeError):
             op(dz, ts)
 
+    @pytest.mark.parametrize('op', [operator.eq, operator.ne,
+                                    operator.gt, operator.ge,
+                                    operator.lt, operator.le])
+    def test_nat_comparison_tzawareness(self, op):
+        # GH#19276
+        # tzaware DatetimeIndex should not raise when compared to NaT
+        dti = pd.DatetimeIndex(['2014-01-01', pd.NaT, '2014-03-01', pd.NaT,
+                                '2014-05-01', '2014-07-01'])
+        expected = np.array([op == operator.ne] * len(dti))
+        result = op(dti, pd.NaT)
+        tm.assert_numpy_array_equal(result, expected)
+
+        result = op(dti.tz_localize('US/Pacific'), pd.NaT)
+        tm.assert_numpy_array_equal(result, expected)
+
     def test_comparisons_coverage(self):
         rng = date_range('1/1/2000', periods=10)