DEPR: Series[td64].fillna(incompat) cast instead of raise (#45746)

Author: jbrockmendel

doc/source/whatsnew/v1.5.0.rst

@@ -193,6 +193,7 @@ Other Deprecations
 - Deprecated :meth:`DataFrame.iteritems`, :meth:`Series.iteritems`, :meth:`HDFStore.iteritems` in favor of :meth:`DataFrame.items`, :meth:`Series.items`, :meth:`HDFStore.items`  (:issue:`45321`)
 - Deprecated :meth:`Series.is_monotonic` and :meth:`Index.is_monotonic` in favor of :meth:`Series.is_monotonic_increasing` and :meth:`Index.is_monotonic_increasing` (:issue:`45422`, :issue:`21335`)
 - Deprecated the ``__array_wrap__`` method of DataFrame and Series, rely on standard numpy ufuncs instead (:issue:`45451`)
+- Deprecated the behavior of :meth:`Series.fillna` and :meth:`DataFrame.fillna` with ``timedelta64[ns]`` dtype and incompatible fill value; in a future version this will cast to a common dtype (usually object) instead of raising, matching the behavior of other dtypes (:issue:`45746`)
 -
 
 

pandas/core/internals/blocks.py

@@ -1488,7 +1488,16 @@ def fillna(
                 # We support filling a DatetimeTZ with a `value` whose timezone
                 #  is different by coercing to object.
                 if self.dtype.kind == "m":
-                    # TODO: don't special-case td64
+                    # 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
                 blk = self.coerce_to_target_dtype(value)
                 return blk.fillna(value, limit, inplace, downcast)

pandas/tests/series/methods/test_fillna.py

@@ -250,8 +250,11 @@ def test_timedelta_fillna(self, frame_or_series):
 
         # 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):
-            obj.fillna(1)
+            with tm.assert_produces_warning(FutureWarning, match=wmsg):
+                # GH#45746
+                obj.fillna(1)
 
         result = obj.fillna(Timedelta(seconds=1))
         expected = Series(