DEPR: Index.ravel behavior

pandas/core/indexes/base.py

@@ -623,7 +623,17 @@ def ravel(self, order="C"):
         --------
         numpy.ndarray.ravel
         """
-        return self._ndarray_values.ravel(order=order)
+        values = self._values
+        if isinstance(values, np.ndarray):
+            return values.ravel(order=order)
+
+        warnings.warn(
+            f"The behavior of Index.ravel for {self.dtype} is deprecated; "
+            "in a future version it will return the underlying ExtensionArray",
+            FutureWarning,
+            stacklevel=2,
+        )
+        return values._ndarray_values.ravel(order=order)
 
     def view(self, cls=None):
 

pandas/tests/indexes/categorical/test_category.py

@@ -645,3 +645,8 @@ def test_reindex_base(self):
     def test_map_str(self):
         # See test_map.py
         pass
+
+    def test_ravel_deprecated(self):
+        idx = self.create_index()
+        with tm.assert_produces_warning(FutureWarning):
+            idx.ravel()

pandas/tests/indexes/datetimelike.py

@@ -95,3 +95,8 @@ def test_map_dictlike(self, mapper):
         expected = pd.Index([np.nan] * len(index))
         result = index.map(mapper([], []))
         tm.assert_index_equal(result, expected)
+
+    def test_ravel_deprecated(self):
+        idx = self.create_index()
+        with tm.assert_produces_warning(FutureWarning):
+            idx.ravel()

pandas/tests/indexes/interval/test_interval.py

@@ -857,6 +857,11 @@ def test_is_all_dates(self):
         year_2017_index = pd.IntervalIndex([year_2017])
         assert not year_2017_index.is_all_dates
 
+    def test_ravel_deprecated(self):
+        idx = self.create_index()
+        with tm.assert_produces_warning(FutureWarning):
+            idx.ravel()
+
 
 def test_dir():
     # GH#27571 dir(interval_index) should not raise