CLN/REF: de-duplicate DatetimeTZBlock.setitem (#37019)

Author: jbrockmendel

pandas/core/internals/blocks.py

@@ -1758,6 +1758,14 @@ def setitem(self, indexer, value):
         `indexer` is a direct slice/positional indexer. `value` must
         be a compatible shape.
         """
+        if not self._can_hold_element(value):
+            # This is only relevant for DatetimeTZBlock, which has a
+            #  non-trivial `_can_hold_element`.
+            # https://github.com/pandas-dev/pandas/issues/24020
+            # Need a dedicated setitem until GH#24020 (type promotion in setitem
+            #  for extension arrays) is designed and implemented.
+            return self.astype(object).setitem(indexer, value)
+
         if isinstance(indexer, tuple):
             # TODO(EA2D): not needed with 2D EAs
             # we are always 1-D
@@ -2175,7 +2183,13 @@ def astype(self, dtype, copy: bool = False, errors: str = "raise"):
     def _can_hold_element(self, element: Any) -> bool:
         tipo = maybe_infer_dtype_type(element)
         if tipo is not None:
-            if self.is_datetimetz:
+            if isinstance(element, list) and len(element) == 0:
+                # Following DatetimeArray._validate_setitem_value
+                #  convention, we treat this as object-dtype
+                #  (even though tipo is float64)
+                return True
+
+            elif self.is_datetimetz:
                 # require exact match, since non-nano does not exist
                 return is_dtype_equal(tipo, self.dtype) or is_valid_nat_for_dtype(
                     element, self.dtype
@@ -2339,21 +2353,6 @@ def fillna(self, value, limit=None, inplace=False, downcast=None):
             value, limit=limit, inplace=inplace, downcast=downcast
         )
 
-    def setitem(self, indexer, value):
-        # https://github.com/pandas-dev/pandas/issues/24020
-        # Need a dedicated setitem until #24020 (type promotion in setitem
-        # for extension arrays) is designed and implemented.
-        if self._can_hold_element(value) or (
-            isinstance(indexer, np.ndarray) and indexer.size == 0
-        ):
-            return super().setitem(indexer, value)
-
-        obj_vals = self.values.astype(object)
-        newb = make_block(
-            obj_vals, placement=self.mgr_locs, klass=ObjectBlock, ndim=self.ndim
-        )
-        return newb.setitem(indexer, value)
-
     def quantile(self, qs, interpolation="linear", axis=0):
         naive = self.values.view("M8[ns]")