BUG: fix+test fillna with non-nano datetime64; closes #27419 (#27425)

Author: jbrockmendel

pandas/core/internals/blocks.py

@@ -2250,7 +2250,12 @@ def _astype(self, dtype, **kwargs):
  def _can_hold_element(self, element):
  tipo = maybe_infer_dtype_type(element)
  if tipo is not None:
- return is_dtype_equal(tipo, self.dtype)
+ if self.is_datetimetz:
+ # require exact match, since non-nano does not exist
+ return is_dtype_equal(tipo, self.dtype)
+
+ # GH#27419 if we get a non-nano datetime64 object
+ return is_datetime64_dtype(tipo)
  elif element is NaT:
  return True
  elif isinstance(element, datetime):

pandas/tests/series/test_missing.py

@@ -437,6 +437,17 @@ def test_datetime64_tz_fillna(self):
  )
  assert_series_equal(df.fillna(method="bfill"), exp)
 
+ def test_datetime64_non_nano_fillna(self):
+ # GH#27419
+ ser = Series([Timestamp("2010-01-01"), pd.NaT, Timestamp("2000-01-01")])
+ val = np.datetime64("1975-04-05", "ms")
+
+ result = ser.fillna(val)
+ expected = Series(
+ [Timestamp("2010-01-01"), Timestamp("1975-04-05"), Timestamp("2000-01-01")]
+ )
+ tm.assert_series_equal(result, expected)
+
  def test_fillna_consistency(self):
  # GH 16402
  # fillna with a tz aware to a tz-naive, should result in object