TYP: is_dtype_compat (#37340)

Author: jbrockmendel

pandas/core/indexes/category.py

@@ -235,20 +235,23 @@ def _shallow_copy(self, values=None, name: Label = no_default):
 
         return super()._shallow_copy(values=values, name=name)
 
-    def _is_dtype_compat(self, other) -> bool:
+    def _is_dtype_compat(self, other) -> Categorical:
         """
         *this is an internal non-public method*
 
         provide a comparison between the dtype of self and other (coercing if
         needed)
 
+        Returns
+        -------
+        Categorical
+
         Raises
         ------
         TypeError if the dtypes are not compatible
         """
         if is_categorical_dtype(other):
-            if isinstance(other, CategoricalIndex):
-                other = other._values
+            other = extract_array(other)
             if not other.is_dtype_equal(self):
                 raise TypeError(
                     "categories must match existing categories when appending"
@@ -263,6 +266,7 @@ def _is_dtype_compat(self, other) -> bool:
                 raise TypeError(
                     "cannot append a non-category item to a CategoricalIndex"
                 )
+            other = other._values
 
         return other