REF: share _maybe_downcast between Block subclasses (#40619)

Author: jbrockmendel

pandas/core/internals/blocks.py

@@ -510,8 +510,19 @@ def split_and_operate(self, func, *args, **kwargs) -> List[Block]:
             res_blocks.extend(rbs)
         return res_blocks
 
+    @final
     def _maybe_downcast(self, blocks: List[Block], downcast=None) -> List[Block]:
 
+        if self.dtype == _dtype_obj:
+            # TODO: why is behavior different for object dtype?
+            if downcast is not None:
+                return blocks
+
+            # split and convert the blocks
+            return extend_blocks(
+                [blk.convert(datetime=True, numeric=False) for blk in blocks]
+            )
+
         # no need to downcast our float
         # unless indicated
         if downcast is None and self.dtype.kind in ["f", "m", "M"]:
@@ -520,6 +531,7 @@ def _maybe_downcast(self, blocks: List[Block], downcast=None) -> List[Block]:
 
         return extend_blocks([b.downcast(downcast) for b in blocks])
 
+    @final
     def downcast(self, dtypes=None) -> List[Block]:
         """ try to downcast each item to the dict of dtypes if present """
         # turn it off completely
@@ -1375,6 +1387,7 @@ def _unstack(self, unstacker, fill_value, new_placement):
         blocks = [new_block(new_values, placement=new_placement, ndim=2)]
         return blocks, mask
 
+    @final
     def quantile(
         self, qs: Float64Index, interpolation="linear", axis: int = 0
     ) -> Block:
@@ -1952,14 +1965,6 @@ def convert(
         res_values = ensure_block_shape(res_values, self.ndim)
         return [self.make_block(res_values)]
 
-    def _maybe_downcast(self, blocks: List[Block], downcast=None) -> List[Block]:
-
-        if downcast is not None:
-            return blocks
-
-        # split and convert the blocks
-        return extend_blocks([b.convert(datetime=True, numeric=False) for b in blocks])
-
     def _can_hold_element(self, element: Any) -> bool:
         return True