DEPR Series[td64].fillna(incompatible) (pandas-dev#49479)

Author: jbrockmendel

doc/source/whatsnew/v2.0.0.rst

@@ -409,6 +409,7 @@ Removal of prior version deprecations/changes
 - Changed behavior of setitem-like operations (``__setitem__``, ``fillna``, ``where``, ``mask``, ``replace``, ``insert``, fill_value for ``shift``) on an object with :class:`DatetimeTZDtype` when using a value with a non-matching timezone, the value will be cast to the object's timezone instead of casting both to object-dtype (:issue:`44243`)
 - Changed behavior of :class:`Index`, :class:`Series`, :class:`DataFrame` constructors with floating-dtype data and a :class:`DatetimeTZDtype`, the data are now interpreted as UTC-times instead of wall-times, consistent with how integer-dtype data are treated (:issue:`45573`)
 - Removed the deprecated ``base`` and ``loffset`` arguments from :meth:`pandas.DataFrame.resample`, :meth:`pandas.Series.resample` and :class:`pandas.Grouper`. Use ``offset`` or ``origin`` instead (:issue:`31809`)
+- Changed behavior of :meth:`Series.fillna` and :meth:`DataFrame.fillna` with ``timedelta64[ns]`` dtype and an incompatible ``fill_value``; this now casts to ``object`` dtype instead of raising, consistent with the behavior with other dtypes (:issue:`45746`)
 - Change the default argument of ``regex`` for :meth:`Series.str.replace` from ``True`` to ``False``. Additionally, a single character ``pat`` with ``regex=True`` is now treated as a regular expression instead of a string literal. (:issue:`36695`, :issue:`24804`)
 - Changed behavior of :meth:`DataFrame.any` and :meth:`DataFrame.all` with ``bool_only=True``; object-dtype columns with all-bool values will no longer be included, manually cast to ``bool`` dtype first (:issue:`46188`)
 - Changed behavior of comparison of a :class:`Timestamp` with a ``datetime.date`` object; these now compare as un-equal and raise on inequality comparisons, matching the ``datetime.datetime`` behavior (:issue:`36131`)

pandas/core/internals/blocks.py

@@ -11,7 +11,6 @@
     cast,
     final,
 )
-import warnings
 
 import numpy as np
 
@@ -36,7 +35,6 @@
 )
 from pandas.errors import AbstractMethodError
 from pandas.util._decorators import cache_readonly
-from pandas.util._exceptions import find_stack_level
 from pandas.util._validators import validate_bool_kwarg
 
 from pandas.core.dtypes.astype import astype_array_safe
@@ -1545,35 +1543,6 @@ def putmask(self, mask, new) -> list[Block]:
 
         return [self]
 
-    def fillna(
-        self, value, limit: int | None = None, inplace: bool = False, downcast=None
-    ) -> list[Block]:
-        # Caller is responsible for validating limit; if int it is strictly positive
-
-        if self.dtype.kind == "m":
-            try:
-                res_values = self.values.fillna(value, limit=limit)
-            except (ValueError, TypeError):
-                # GH#45746
-                warnings.warn(
-                    "The behavior of fillna with timedelta64[ns] dtype and "
-                    f"an incompatible value ({type(value)}) is deprecated. "
-                    "In a future version, this will cast to a common dtype "
-                    "(usually object) instead of raising, matching the "
-                    "behavior of other dtypes.",
-                    FutureWarning,
-                    stacklevel=find_stack_level(),
-                )
-                raise
-            else:
-                res_blk = self.make_block(res_values)
-                return [res_blk]
-
-        # TODO: since this now dispatches to super, which in turn dispatches
-        #  to putmask, it may *actually* respect 'inplace=True'. If so, add
-        #  tests for this.
-        return super().fillna(value, limit=limit, inplace=inplace, downcast=downcast)
-
     def delete(self, loc) -> Block:
         # This will be unnecessary if/when __array_function__ is implemented
         values = self.values.delete(loc)

pandas/tests/series/methods/test_fillna.py

@@ -243,13 +243,12 @@ def test_timedelta_fillna(self, frame_or_series):
         expected = frame_or_series(expected)
         tm.assert_equal(result, expected)
 
-        # interpreted as seconds, no longer supported
-        msg = "value should be a 'Timedelta', 'NaT', or array of those. Got 'int'"
-        wmsg = "In a future version, this will cast to a common dtype"
-        with pytest.raises(TypeError, match=msg):
-            with tm.assert_produces_warning(FutureWarning, match=wmsg):
-                # GH#45746
-                obj.fillna(1)
+        # GH#45746 pre-1.? ints were interpreted as seconds.  then that was
+        #  deprecated and changed to raise. In 2.0 it casts to common dtype,
+        #  consistent with every other dtype's behavior
+        res = obj.fillna(1)
+        expected = obj.astype(object).fillna(1)
+        tm.assert_equal(res, expected)
 
         result = obj.fillna(Timedelta(seconds=1))
         expected = Series(