REF: remove Block.putmask fallback to where (pandas-dev#45440)

Author: jbrockmendel

pandas/core/array_algos/putmask.py

@@ -126,6 +126,8 @@ def putmask_without_repeat(
     mask : np.ndarray[bool]
     new : Any
     """
+    new = setitem_datetimelike_compat(values, mask.sum(), new)
+
     if getattr(new, "ndim", 0) >= 1:
         new = new.astype(values.dtype, copy=False)
 

pandas/core/internals/blocks.py

@@ -30,7 +30,6 @@
     Shape,
     npt,
 )
-from pandas.compat import np_version_under1p20
 from pandas.util._decorators import cache_readonly
 from pandas.util._exceptions import find_stack_level
 from pandas.util._validators import validate_bool_kwarg
@@ -39,7 +38,6 @@
 from pandas.core.dtypes.cast import (
     can_hold_element,
     find_result_type,
-    infer_dtype_from,
     maybe_downcast_numeric,
     maybe_downcast_to_dtype,
     soft_convert_objects,
@@ -977,12 +975,6 @@ def putmask(self, mask, new) -> list[Block]:
             putmask_without_repeat(values.T, mask, new)
             return [self]
 
-        elif np_version_under1p20 and infer_dtype_from(new)[0].kind in ["m", "M"]:
-            # using putmask with object dtype will incorrectly cast to object
-            # Having excluded self._can_hold_element, we know we cannot operate
-            #  in-place, so we are safe using `where`
-            return self.where(new, ~mask)
-
         elif noop:
             return [self]