fix+test to_timedelta('NaT', box=False) (#24961)

Author: jbrockmendel

doc/source/whatsnew/v0.24.1.rst

@@ -66,7 +66,7 @@ Bug Fixes
 -
 
 **Timedelta**
-
+- Bug in :func:`to_timedelta` with `box=False` incorrectly returning a ``datetime64`` object instead of a ``timedelta64`` object (:issue:`24961`)
 -
 -
 -

pandas/core/tools/timedeltas.py

@@ -120,7 +120,8 @@ def _coerce_scalar_to_timedelta_type(r, unit='ns', box=True, errors='raise'):
  try:
  result = Timedelta(r, unit)
  if not box:
- result = result.asm8
+ # explicitly view as timedelta64 for case when result is pd.NaT
+ result = result.asm8.view('timedelta64[ns]')
  except ValueError:
  if errors == 'raise':
  raise

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -309,8 +309,13 @@ def test_iso_conversion(self):
  assert to_timedelta('P0DT0H0M1S') == expected
 
  def test_nat_converters(self):
- assert to_timedelta('nat', box=False).astype('int64') == iNaT
- assert to_timedelta('nan', box=False).astype('int64') == iNaT
+ result = to_timedelta('nat', box=False)
+ assert result.dtype.kind == 'm'
+ assert result.astype('int64') == iNaT
+
+ result = to_timedelta('nan', box=False)
+ assert result.dtype.kind == 'm'
+ assert result.astype('int64') == iNaT
 
  @pytest.mark.parametrize('units, np_unit',
  [(['Y', 'y'], 'Y'),