Improve code to handle it in the original conditional

Author: jmarin

pandas/core/indexes/base.py

@@ -2916,15 +2916,23 @@ def union(self, other, sort=None):
         self._assert_can_do_setop(other)
         other, result_name = self._convert_can_do_setop(other)
 
-        if isinstance(self.dtype, CategoricalDtype) and isinstance(
-            other.dtype, CategoricalDtype
-        ):
-            both_categories = self.categories  # type: ignore[attr-defined]
-            # if ordered and unordered, we set categories to be unordered
-            ordered = False if self.ordered != other.ordered else None  # type: ignore[attr-defined]
-            if len(self.categories) != len(other.categories) or any(  # type: ignore[attr-defined]
-                self.categories != other.categories  # type: ignore[attr-defined]
+        if self.dtype != other.dtype:
+            if (
+                isinstance(self, ABCMultiIndex)
+                and not is_object_dtype(_unpack_nested_dtype(other))
+                and len(other) > 0
+            ):
+                raise NotImplementedError(
+                    "Can only union MultiIndex with MultiIndex or Index of tuples, "
+                    "try mi.to_flat_index().union(other) instead."
+                )
+
+            if isinstance(self, ABCCategoricalIndex) and isinstance(
+                other, ABCCategoricalIndex
             ):
+                both_categories = self.categories
+                # if ordered and unordered, we set categories to be unordered
+                ordered = False if self.ordered != other.ordered else None
                 if ordered is False:
                     both_categories = union_categoricals(
                         [self.as_unordered(), other.as_unordered()],  # type: ignore[attr-defined]
@@ -2934,20 +2942,11 @@ def union(self, other, sort=None):
                     both_categories = union_categoricals(
                         [self, other], sort_categories=True
                     ).categories
-            # Convert both indexes to have the same categories
-            self = self.set_categories(both_categories, ordered=ordered)  # type: ignore[attr-defined]
-            other = other.set_categories(both_categories, ordered=ordered)
+                # Convert both indexes to have the same categories
+                self = self.set_categories(both_categories, ordered=ordered)  # type: ignore[attr-defined]
+                other = other.set_categories(both_categories, ordered=ordered)  # type: ignore[attr-defined]
+                return self.union(other, sort=sort)
 
-        elif self.dtype != other.dtype:
-            if (
-                isinstance(self, ABCMultiIndex)
-                and not is_object_dtype(_unpack_nested_dtype(other))
-                and len(other) > 0
-            ):
-                raise NotImplementedError(
-                    "Can only union MultiIndex with MultiIndex or Index of tuples, "
-                    "try mi.to_flat_index().union(other) instead."
-                )
             self, other = self._dti_setop_align_tzs(other, "union")
 
             dtype = self._find_common_type_compat(other)