COMPAT: add back dummy CategoricalBlock class (#40582)

pandas/core/internals/__init__.py

@@ -26,6 +26,7 @@
 
 __all__ = [
     "Block",
+    "CategoricalBlock",
     "NumericBlock",
     "DatetimeBlock",
     "DatetimeTZBlock",
@@ -56,6 +57,8 @@ def __getattr__(name: str):
             DeprecationWarning,
             stacklevel=2,
         )
-        return ExtensionBlock
+        from pandas.core.internals.blocks import CategoricalBlock
+
+        return CategoricalBlock
 
     raise AttributeError(f"module 'pandas.core.internals' has no attribute '{name}'")

pandas/core/internals/blocks.py

@@ -1244,7 +1244,7 @@ def take_nd(
         Take values according to indexer and return them as a block.bb
 
         """
-        # algos.take_nd dispatches for DatetimeTZBlock
+        # algos.take_nd dispatches for DatetimeTZBlock, CategoricalBlock
         # so need to preserve types
         # sparse is treated like an ndarray, but needs .get_values() shaping
 
@@ -1443,7 +1443,7 @@ class ExtensionBlock(Block):
     Notes
     -----
     This holds all 3rd-party extension array types. It's also the immediate
-    parent class for our internal extension types' blocks.
+    parent class for our internal extension types' blocks, CategoricalBlock.
 
     ExtensionArrays are limited to 1-D.
     """
@@ -2017,6 +2017,11 @@ def _can_hold_element(self, element: Any) -> bool:
         return True
 
 
+class CategoricalBlock(ExtensionBlock):
+    # this Block type is kept for backwards-compatibility
+    __slots__ = ()
+
+
 # -----------------------------------------------------------------
 # Constructor Helpers
 
@@ -2078,7 +2083,7 @@ def get_block_type(values, dtype: Optional[Dtype] = None):
         # Need this first(ish) so that Sparse[datetime] is sparse
         cls = ExtensionBlock
     elif isinstance(dtype, CategoricalDtype):
-        cls = ExtensionBlock
+        cls = CategoricalBlock
     elif vtype is Timestamp:
         cls = DatetimeTZBlock
     elif vtype is Interval or vtype is Period:

pandas/core/internals/managers.py

@@ -67,6 +67,7 @@
 )
 from pandas.core.internals.blocks import (
     Block,
+    CategoricalBlock,
     DatetimeTZBlock,
     ExtensionBlock,
     ObjectValuesExtensionBlock,
@@ -1860,6 +1861,13 @@ def _form_blocks(
         object_blocks = _simple_blockify(items_dict["ObjectBlock"], np.object_)
         blocks.extend(object_blocks)
 
+    if len(items_dict["CategoricalBlock"]) > 0:
+        cat_blocks = [
+            new_block(array, klass=CategoricalBlock, placement=i, ndim=2)
+            for i, array in items_dict["CategoricalBlock"]
+        ]
+        blocks.extend(cat_blocks)
+
     if len(items_dict["ExtensionBlock"]):
         external_blocks = [
             new_block(array, klass=ExtensionBlock, placement=i, ndim=2)