From 605cc0b8d08fb68fcf0d58fadb6aeaef10061340 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Mon, 5 Oct 2020 20:40:18 -0700
Subject: [PATCH] DEPR: Index.ravel returning an ndarray (#36900)

---
 doc/source/whatsnew/v1.2.0.rst      | 1 +
 pandas/core/indexes/base.py         | 6 ++++++
 pandas/io/formats/format.py         | 3 ++-
 pandas/tests/indexes/test_common.py | 5 +++++
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 95628350ad998..7dd3eb51bcaeb 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -268,6 +268,7 @@ Deprecations
 - Deprecated :meth:`Index.is_all_dates` (:issue:`27744`)
 - Deprecated automatic alignment on comparison operations between :class:`DataFrame` and :class:`Series`, do ``frame, ser = frame.align(ser, axis=1, copy=False)`` before e.g. ``frame == ser`` (:issue:`28759`)
 - :meth:`Rolling.count` with ``min_periods=None`` will default to the size of the window in a future version (:issue:`31302`)
+- :meth:`Index.ravel` returning a ``np.ndarray`` is deprecated, in the future this will return a view on the same index (:issue:`19956`)
 
 .. ---------------------------------------------------------------------------
 
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index ff3d8bf05f9a5..d603797370ce3 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -659,6 +659,12 @@ def ravel(self, order="C"):
         --------
         numpy.ndarray.ravel
         """
+        warnings.warn(
+            "Index.ravel returning ndarray is deprecated; in a future version "
+            "this will return a view on self.",
+            FutureWarning,
+            stacklevel=2,
+        )
         values = self._get_engine_target()
         return values.ravel(order=order)
 
diff --git a/pandas/io/formats/format.py b/pandas/io/formats/format.py
index b9d41f142c2b5..13010bb2ef147 100644
--- a/pandas/io/formats/format.py
+++ b/pandas/io/formats/format.py
@@ -1677,7 +1677,8 @@ def is_dates_only(
     values: Union[np.ndarray, DatetimeArray, Index, DatetimeIndex]
 ) -> bool:
     # return a boolean if we are only dates (and don't have a timezone)
-    values = values.ravel()
+    if not isinstance(values, Index):
+        values = values.ravel()
 
     values = DatetimeIndex(values)
     if values.tz is not None:
diff --git a/pandas/tests/indexes/test_common.py b/pandas/tests/indexes/test_common.py
index 675ae388a28a4..e2dea7828b3ad 100644
--- a/pandas/tests/indexes/test_common.py
+++ b/pandas/tests/indexes/test_common.py
@@ -399,6 +399,11 @@ def test_astype_preserves_name(self, index, dtype):
         else:
             assert result.name == index.name
 
+    def test_ravel_deprecation(self, index):
+        # GH#19956 ravel returning ndarray is deprecated
+        with tm.assert_produces_warning(FutureWarning):
+            index.ravel()
+
 
 @pytest.mark.parametrize("na_position", [None, "middle"])
 def test_sort_values_invalid_na_position(index_with_missing, na_position):