From e1d9a12f94aaa91f63e3352eb4e4e649d9d5b04b Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Wed, 11 Mar 2020 16:30:28 -0700
Subject: [PATCH] DEPR: Index.ravel behavior

---
 pandas/core/indexes/base.py                       | 12 +++++++++++-
 pandas/tests/indexes/categorical/test_category.py |  5 +++++
 pandas/tests/indexes/datetimelike.py              |  5 +++++
 pandas/tests/indexes/interval/test_interval.py    |  5 +++++
 4 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 98e3b3ad258ea..c7fbb4aeca493 100644
--- a/pandas/core/indexes/base.py
+++ b/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):
 
diff --git a/pandas/tests/indexes/categorical/test_category.py b/pandas/tests/indexes/categorical/test_category.py
index c18cd1f252c83..475d3c1642062 100644
--- a/pandas/tests/indexes/categorical/test_category.py
+++ b/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()
diff --git a/pandas/tests/indexes/datetimelike.py b/pandas/tests/indexes/datetimelike.py
index ba10976a67e9a..0655424d67f95 100644
--- a/pandas/tests/indexes/datetimelike.py
+++ b/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()
diff --git a/pandas/tests/indexes/interval/test_interval.py b/pandas/tests/indexes/interval/test_interval.py
index c2b209c810af9..cbf3ae556e795 100644
--- a/pandas/tests/indexes/interval/test_interval.py
+++ b/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