short circut only for the dtype

Author: mroeschke

pandas/core/dtypes/dtypes.py

@@ -611,7 +611,11 @@ def update_dtype(self, dtype: str_type | CategoricalDtype) -> CategoricalDtype:
             dtype = cast(CategoricalDtype, dtype)
 
         # update categories/ordered unless they've been explicitly passed as None
-        if dtype.categories is not None and dtype.ordered is not None:
+        if (
+            isinstance(dtype, CategoricalDtype)
+            and dtype.categories is not None
+            and dtype.ordered is not None
+        ):
             # Avoid re-validation in CategoricalDtype constructor
             return dtype
         new_categories = (

pandas/tests/dtypes/test_dtypes.py

@@ -222,9 +222,7 @@ def test_repr_range_categories(self):
 
     def test_update_dtype(self):
         # GH 27338
-        result = CategoricalDtype(["a"]).update_dtype(
-            CategoricalDtype(["b"], ordered=True)
-        )
+        result = CategoricalDtype(["a"]).update_dtype(Categorical(["b"], ordered=True))
         expected = CategoricalDtype(["b"], ordered=True)
         assert result == expected