BUG: Fix inserting of wrong-dtyped NaT, closes #27297

doc/source/whatsnew/v1.0.0.rst

@@ -87,7 +87,7 @@ Categorical
 
 Datetimelike
 ^^^^^^^^^^^^
-
+- Bug in :meth:`Series.__setitem__` incorrectly casting ``np.timedelta64("NaT")`` to ``np.datetime64("NaT")`` when inserting into a :class:`Series` with datetime64 dtype (:issue:`27311`)
 -
 -
 

pandas/_libs/index.pyx

@@ -533,6 +533,9 @@ cpdef convert_scalar(ndarray arr, object value):
             pass
         elif isinstance(value, (datetime, np.datetime64, date)):
             return Timestamp(value).value
+        elif util.is_timedelta64_object(value):
+            # exclude np.timedelta64("NaT") from value != value below
+            pass
         elif value is None or value != value:
             return NPY_NAT
         elif isinstance(value, str):

pandas/core/internals/blocks.py

@@ -2589,8 +2589,9 @@ def setitem(self, indexer, value):
         try:
             return super().setitem(indexer, value)
         except (ValueError, TypeError):
+            obj_vals = self.values.astype(object)
             newb = make_block(
-                self.values.astype(object), placement=self.mgr_locs, klass=ObjectBlock
+                obj_vals, placement=self.mgr_locs, klass=ObjectBlock, ndim=self.ndim
             )
             return newb.setitem(indexer, value)
 

pandas/tests/series/indexing/test_indexing.py

@@ -654,6 +654,38 @@ def test_timedelta_assignment():
     tm.assert_series_equal(s, expected)
 
 
+@pytest.mark.parametrize(
+    "nat_val,should_cast",
+    [
+        (pd.NaT, True),
+        (np.timedelta64("NaT", "ns"), False),
+        (np.datetime64("NaT", "ns"), True),
+    ],
+)
+@pytest.mark.parametrize("tz", [None, "UTC"])
+def test_dt64_series_assign_nat(nat_val, should_cast, tz):
+    # some nat-like values should be cast to datetime64 when inserting
+    #  into a datetime64 series.  Others should coerce to object
+    #  and retain their dtypes.
+    dti = pd.date_range("2016-01-01", periods=3, tz=tz)
+    base = pd.Series(dti)
+    expected = pd.Series([pd.NaT] + list(dti[1:]), dtype=dti.dtype)
+    if not should_cast:
+        expected = expected.astype(object)
+
+    ser = base.copy(deep=True)
+    ser[0] = nat_val
+    tm.assert_series_equal(ser, expected)
+
+    ser = base.copy(deep=True)
+    ser.loc[0] = nat_val
+    tm.assert_series_equal(ser, expected)
+
+    ser = base.copy(deep=True)
+    ser.iloc[0] = nat_val
+    tm.assert_series_equal(ser, expected)
+
+
 @pytest.mark.parametrize(
     "nat_val,should_cast",
     [