From b4755e4fdb940f11f6aa22c1c722c1602c8e6b03 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Wed, 8 Nov 2023 21:52:09 +0000
Subject: [PATCH] set full column if using self.index in loc

---
 pandas/core/indexing.py                     | 3 +++
 pandas/tests/frame/indexing/test_setitem.py | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 13756dd5a70e4..8a9880354a24a 100644
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -1454,6 +1454,9 @@ def _convert_to_indexer(self, key, axis: AxisInt):
         if isinstance(key, slice):
             return labels._convert_slice_indexer(key, kind="loc")
 
+        if key is self.obj.index:
+            return slice(None, None, None)
+
         if (
             isinstance(key, tuple)
             and not isinstance(labels, MultiIndex)
diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index 49dd7a3c4df9b..9f79fa3c59caa 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -1351,3 +1351,11 @@ def test_frame_setitem_empty_dataframe(self):
             index=Index([], dtype="datetime64[ns]", name="date"),
         )
         tm.assert_frame_equal(df, expected)
+
+    def test_setitem_frame_with_dataframes_own_index(self):
+        # https://github.com/pandas-dev/pandas/issues/55791
+        df1 = DataFrame({"a": [1]})
+        df2 = DataFrame({"d": [True]})
+        df1.loc[df1.index, df2.columns] = df2
+        expected = DataFrame({"a": [1], "d": [True]})
+        tm.assert_frame_equal(df1, expected)