REGR: comparing datetime vs None raises (#54880)

Author: phofl

doc/source/whatsnew/v2.1.1.rst

@@ -15,6 +15,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed regression in :func:`read_csv` when ``usecols`` is given and ``dtypes`` is a dict for ``engine="python"`` (:issue:`54868`)
 - Fixed regression in :meth:`DataFrame.__setitem__` raising ``AssertionError`` when setting a :class:`Series` with a partial :class:`MultiIndex` (:issue:`54875`)
+- Fixed regression when comparing a :class:`Series` with ``datetime64`` dtype with ``None`` (:issue:`54870`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_211.bug_fixes:

pandas/core/arrays/datetimelike.py

@@ -107,6 +107,7 @@
     algorithms,
     missing,
     nanops,
+    ops,
 )
 from pandas.core.algorithms import (
     checked_add_with_arr,
@@ -947,8 +948,12 @@ def _cmp_method(self, other, op):
 
         dtype = getattr(other, "dtype", None)
         if is_object_dtype(dtype):
-            return op(np.asarray(self, dtype=object), other)
-
+            # We have to use comp_method_OBJECT_ARRAY instead of numpy
+            #  comparison otherwise it would raise when comparing to None
+            result = ops.comp_method_OBJECT_ARRAY(
+                op, np.asarray(self.astype(object)), other
+            )
+            return result
         if other is NaT:
             if op is operator.ne:
                 result = np.ones(self.shape, dtype=bool)

pandas/tests/series/indexing/test_datetime.py

@@ -486,3 +486,12 @@ def test_getitem_str_second_with_datetimeindex():
     msg = r"Timestamp\('2012-01-02 18:01:02-0600', tz='US/Central'\)"
     with pytest.raises(KeyError, match=msg):
         df[df.index[2]]
+
+
+def test_compare_datetime_with_all_none():
+    # GH#54870
+    ser = Series(["2020-01-01", "2020-01-02"], dtype="datetime64[ns]")
+    ser2 = Series([None, None])
+    result = ser > ser2
+    expected = Series([False, False])
+    tm.assert_series_equal(result, expected)