BUG: disallow invalid dtype to CategoricalDtype._from_values_or_dtype (#32169)

Author: jbrockmendel

pandas/core/dtypes/dtypes.py

@@ -324,6 +324,8 @@ def _from_values_or_dtype(
                 raise ValueError(
                     "Cannot specify `categories` or `ordered` together with `dtype`."
                 )
+            elif not isinstance(dtype, CategoricalDtype):
+                raise ValueError(f"Cannot not construct CategoricalDtype from {dtype}")
         elif is_categorical(values):
             # If no "dtype" was passed, use the one from "values", but honor
             # the "ordered" and "categories" arguments

pandas/tests/dtypes/test_dtypes.py

@@ -127,6 +127,11 @@ def test_from_values_or_dtype_raises(self, values, categories, ordered, dtype):
         with pytest.raises(ValueError, match=msg):
             CategoricalDtype._from_values_or_dtype(values, categories, ordered, dtype)
 
+    def test_from_values_or_dtype_invalid_dtype(self):
+        msg = "Cannot not construct CategoricalDtype from <class 'object'>"
+        with pytest.raises(ValueError, match=msg):
+            CategoricalDtype._from_values_or_dtype(None, None, None, object)
+
     def test_is_dtype(self, dtype):
         assert CategoricalDtype.is_dtype(dtype)
         assert CategoricalDtype.is_dtype("category")

pandas/tests/indexes/common.py

@@ -605,7 +605,8 @@ def test_equals(self, indices):
         assert not indices.equals(np.array(indices))
 
         # Cannot pass in non-int64 dtype to RangeIndex
-        if not isinstance(indices, RangeIndex):
+        if not isinstance(indices, (RangeIndex, CategoricalIndex)):
+            # TODO: CategoricalIndex can be re-allowed following GH#32167
             same_values = Index(indices, dtype=object)
             assert indices.equals(same_values)
             assert same_values.equals(indices)