REF: remove CategoricalIndex._reindex_non_unique (#42094)

Author: jbrockmendel

pandas/core/indexes/base.py

@@ -3794,6 +3794,7 @@ def reindex(
 
         return target, indexer
 
+    @final
     def _reindex_non_unique(
         self, target: Index
     ) -> tuple[Index, np.ndarray, np.ndarray | None]:
@@ -3825,14 +3826,15 @@ def _reindex_non_unique(
         new_indexer = None
 
         if len(missing):
-            length = np.arange(len(indexer))
+            length = np.arange(len(indexer), dtype=np.intp)
 
             missing = ensure_platform_int(missing)
             missing_labels = target.take(missing)
-            missing_indexer = ensure_platform_int(length[~check])
+            missing_indexer = length[~check]
             cur_labels = self.take(indexer[check]).values
-            cur_indexer = ensure_platform_int(length[check])
+            cur_indexer = length[check]
 
+            # Index constructor below will do inference
             new_labels = np.empty((len(indexer),), dtype=object)
             new_labels[cur_indexer] = cur_labels
             new_labels[missing_indexer] = missing_labels

pandas/core/indexes/category.py

@@ -442,37 +442,6 @@ def reindex(
 
         return new_target, indexer
 
-    # error: Return type "Tuple[Index, Optional[ndarray], Optional[ndarray]]"
-    # of "_reindex_non_unique" incompatible with return type
-    # "Tuple[Index, ndarray, Optional[ndarray]]" in supertype "Index"
-    def _reindex_non_unique(  # type: ignore[override]
-        self, target: Index
-    ) -> tuple[Index, np.ndarray | None, np.ndarray | None]:
-        """
-        reindex from a non-unique; which CategoricalIndex's are almost
-        always
-        """
-        # TODO: rule out `indexer is None` here to make the signature
-        #  match the parent class's signature. This should be equivalent
-        #  to ruling out `self.equals(target)`
-        new_target, indexer = self.reindex(target)
-        new_indexer = None
-
-        check = indexer == -1
-        # error: Item "bool" of "Union[Any, bool]" has no attribute "any"
-        if check.any():  # type: ignore[union-attr]
-            new_indexer = np.arange(len(self.take(indexer)), dtype=np.intp)
-            new_indexer[check] = -1
-
-        cats = self.categories.get_indexer(target)
-        if not (cats == -1).any():
-            # .reindex returns normal Index. Revert to CategoricalIndex if
-            # all targets are included in my categories
-            cat = Categorical(new_target, dtype=self.dtype)
-            new_target = type(self)._simple_new(cat, name=self.name)
-
-        return new_target, indexer, new_indexer
-
     # --------------------------------------------------------------------
     # Indexing Methods