DEPR. Change RangeIndex._start/_stop/_step to FutureWarning (pandas-dev#30482)

* DEPR. Change RangeIndex.start/stop/step to FutureWarning

* Add whatsnew

* Modify wording in whatsnew

Author: mroeschke

doc/source/whatsnew/v1.0.0.rst

@@ -503,7 +503,7 @@ Deprecations
   Usage of ``json_normalize`` as ``pandas.io.json.json_normalize`` is now deprecated and
   it is recommended to use ``json_normalize`` as :func:`pandas.json_normalize` instead (:issue:`27586`).
 - :meth:`DataFrame.to_stata`, :meth:`DataFrame.to_feather`, and :meth:`DataFrame.to_parquet` argument "fname" is deprecated, use "path" instead (:issue:`23574`)
-
+- The deprecated internal attributes ``_start``, ``_stop`` and ``_step`` of :class:`RangeIndex` now raise a ``FutureWarning`` instead of a ``DeprecationWarning`` (:issue:`26581`)
 
 .. _whatsnew_1000.prior_deprecations:
 

pandas/core/indexes/range.py

@@ -225,7 +225,7 @@ def _start(self):
         """
         warnings.warn(
             self._deprecation_message.format("_start", "start"),
-            DeprecationWarning,
+            FutureWarning,
             stacklevel=2,
         )
         return self.start
@@ -248,7 +248,7 @@ def _stop(self):
         # GH 25710
         warnings.warn(
             self._deprecation_message.format("_stop", "stop"),
-            DeprecationWarning,
+            FutureWarning,
             stacklevel=2,
         )
         return self.stop
@@ -272,7 +272,7 @@ def _step(self):
         # GH 25710
         warnings.warn(
             self._deprecation_message.format("_step", "step"),
-            DeprecationWarning,
+            FutureWarning,
             stacklevel=2,
         )
         return self.step

pandas/tests/indexes/ranges/test_range.py

@@ -64,7 +64,7 @@ def test_start_stop_step_attrs(self, index, start, stop, step):
     def test_deprecated_start_stop_step_attrs(self, attr_name):
         # GH 26581
         idx = self.create_index()
-        with tm.assert_produces_warning(DeprecationWarning):
+        with tm.assert_produces_warning(FutureWarning):
             getattr(idx, attr_name)
 
     def test_copy(self):