BUG: reindex with expansion and non-nanosecond dtype

doc/source/whatsnew/v2.0.3.rst

@@ -22,6 +22,7 @@ Fixed regressions
 Bug fixes
 ~~~~~~~~~
 - Bug in :func:`RangeIndex.union` when using ``sort=True`` with another :class:`RangeIndex` (:issue:`53490`)
+- Bug in :func:`Series.reindex` when expanding a non-nanosecond datetime or timedelta :class:`Series` would not fill with ``NaT`` correctly (:issue:`53497`)
 - Bug in :func:`read_csv` when defining ``dtype`` with ``bool[pyarrow]`` for the ``"c"`` and ``"python"`` engines (:issue:`53390`)
 - Bug in :meth:`Series.str.split` and :meth:`Series.str.rsplit` with ``expand=True`` for :class:`ArrowDtype` with ``pyarrow.string`` (:issue:`53532`)
 -

pandas/core/dtypes/cast.py

@@ -562,9 +562,16 @@ def maybe_promote(dtype: np.dtype, fill_value=np.nan):
         If fill_value is a non-scalar and dtype is not object.
     """
     orig = fill_value
+    orig_is_nat = False
     if checknull(fill_value):
         # https://github.com/pandas-dev/pandas/pull/39692#issuecomment-1441051740
         #  avoid cache misses with NaN/NaT values that are not singletons
+        if fill_value is not NA:
+            try:
+                orig_is_nat = np.isnat(fill_value)
+            except TypeError:
+                pass
+
         fill_value = _canonical_nans.get(type(fill_value), fill_value)
 
     # for performance, we are using a cached version of the actual implementation
@@ -580,8 +587,10 @@ def maybe_promote(dtype: np.dtype, fill_value=np.nan):
         # if fill_value is not hashable (required for caching)
         dtype, fill_value = _maybe_promote(dtype, fill_value)
 
-    if dtype == _dtype_obj and orig is not None:
-        # GH#51592 restore our potentially non-canonical fill_value
+    if (dtype == _dtype_obj and orig is not None) or (
+        orig_is_nat and np.datetime_data(orig)[0] != "ns"
+    ):
+        # GH#51592,53497 restore our potentially non-canonical fill_value
         fill_value = orig
     return dtype, fill_value
 

pandas/tests/series/methods/test_reindex.py

@@ -12,6 +12,7 @@
     NaT,
     Period,
     PeriodIndex,
+    RangeIndex,
     Series,
     Timedelta,
     Timestamp,
@@ -422,3 +423,14 @@ def test_reindexing_with_float64_NA_log():
         result_log = np.log(s_reindex)
         expected_log = Series([0, np.NaN, np.NaN], dtype=Float64Dtype())
         tm.assert_series_equal(result_log, expected_log)
+
+
+@pytest.mark.parametrize("dtype", ["timedelta64", "datetime64"])
+def test_reindex_expand_nonnano_nat(dtype):
+    # GH 53497
+    ser = Series(np.array([1], dtype=f"{dtype}[s]"))
+    result = ser.reindex(RangeIndex(2))
+    expected = Series(
+        np.array([1, getattr(np, dtype)("nat", "s")], dtype=f"{dtype}[s]")
+    )
+    tm.assert_series_equal(result, expected)