BUG: CategoricalDtype.__repr__ with RangeIndex categories (#38245)

* BUG: CategoricalDtype.__repr__ with RangeIndex categories

* fix broken tests

* fixup

Author: jbrockmendel

pandas/core/dtypes/dtypes.py

@@ -399,10 +399,14 @@ def __eq__(self, other: Any) -> bool:
 
     def __repr__(self) -> str_type:
         if self.categories is None:
-            data = "None, "
+            data = "None"
         else:
             data = self.categories._format_data(name=type(self).__name__)
-        return f"CategoricalDtype(categories={data}ordered={self.ordered})"
+            if data is None:
+                # self.categories is RangeIndex
+                data = str(self.categories._range)
+            data = data.rstrip(", ")
+        return f"CategoricalDtype(categories={data}, ordered={self.ordered})"
 
     @staticmethod
     def _hash_categories(categories, ordered: Ordered = True) -> int:

pandas/tests/dtypes/test_dtypes.py

@@ -199,6 +199,14 @@ def test_not_string(self):
         # though CategoricalDtype has object kind, it cannot be string
         assert not is_string_dtype(CategoricalDtype())
 
+    def test_repr_range_categories(self):
+        rng = pd.Index(range(3))
+        dtype = CategoricalDtype(categories=rng, ordered=False)
+        result = repr(dtype)
+
+        expected = "CategoricalDtype(categories=range(0, 3), ordered=False)"
+        assert result == expected
+
 
 class TestDatetimeTZDtype(Base):
     @pytest.fixture