From 9df9c89963e045f13d8cb9da8cee14b9912ec519 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Wed, 4 Nov 2020 00:06:07 +0100
Subject: [PATCH 01/17] Fix bug in setitem when indexer was only false changed
 dtype

---
 doc/source/whatsnew/v1.2.0.rst              | 1 +
 pandas/core/internals/blocks.py             | 3 ++-
 pandas/tests/frame/indexing/test_setitem.py | 7 +++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 7111d54d65815..4d9be2b9d714a 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -457,6 +457,7 @@ Indexing
 - Bug in indexing on a :class:`Series` or :class:`DataFrame` with a :class:`MultiIndex` with a level named "0" (:issue:`37194`)
 - Bug in :meth:`Series.__getitem__` when using an unsigned integer array as an indexer giving incorrect results or segfaulting instead of raising ``KeyError`` (:issue:`37218`)
 - Bug in :meth:`Index.where` incorrectly casting numeric values to strings (:issue:`37591`)
+- Bug in :meth:`DataFrame.loc.__setitem__` changed dtype when indexer was completely ``False``(:issue:`37550`)
 
 Missing
 ^^^^^^^
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 24b00199611bf..810223971ec38 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -919,10 +919,11 @@ def setitem(self, indexer, value):
 
             elif lib.is_scalar(value) and not isna(value):
                 dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
-
             else:
                 # e.g. we are bool dtype and value is nan
                 # TODO: watch out for case with listlike value and scalar/empty indexer
+                if is_list_like(value) and is_empty_indexer(indexer, np.array(value)):
+                    return self
                 dtype, _ = maybe_promote(np.array(value).dtype)
                 return self.astype(dtype).setitem(indexer, value)
 
diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index e1ce10970f07b..41881b628e48d 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -285,3 +285,10 @@ def test_iloc_setitem_bool_indexer(self, klass):
         df.iloc[indexer, 1] = df.iloc[indexer, 1] * 2
         expected = DataFrame({"flag": ["x", "y", "z"], "value": [2, 3, 4]})
         tm.assert_frame_equal(df, expected)
+
+    def test_setitem_only_false_indexer_dtype_changed(self):
+        # GH: 37550
+        df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
+        df.loc[[False], ["b"]] = 10 - df["c"]
+        expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
+        tm.assert_frame_equal(df, expected)

From 26b7dae23aab2a1a71a934474ab50d5aa151f0a0 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Fri, 6 Nov 2020 22:22:28 +0100
Subject: [PATCH 02/17] Add space

---
 doc/source/whatsnew/v1.2.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 13747ffd48303..13409148047c2 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -465,7 +465,7 @@ Indexing
 - Bug in indexing on a :class:`Series` or :class:`DataFrame` with a :class:`MultiIndex` with a level named "0" (:issue:`37194`)
 - Bug in :meth:`Series.__getitem__` when using an unsigned integer array as an indexer giving incorrect results or segfaulting instead of raising ``KeyError`` (:issue:`37218`)
 - Bug in :meth:`Index.where` incorrectly casting numeric values to strings (:issue:`37591`)
-- Bug in :meth:`DataFrame.loc.__setitem__` changed dtype when indexer was completely ``False``(:issue:`37550`)
+- Bug in :meth:`DataFrame.loc.__setitem__` changed dtype when indexer was completely ``False`` (:issue:`37550`)
 
 Missing
 ^^^^^^^

From 11d27b1aac5cbffa7bc88fbe66c23cb4a95e0379 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 8 Nov 2020 12:09:25 +0100
Subject: [PATCH 03/17] Move condition to elif

---
 pandas/core/internals/blocks.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index a14e977beedac..67913b9315d8c 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -920,11 +920,11 @@ def setitem(self, indexer, value):
 
             elif lib.is_scalar(value) and not isna(value):
                 dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
+            elif is_list_like(value) and is_empty_indexer(indexer, np.array(value)):
+                return self
             else:
                 # e.g. we are bool dtype and value is nan
-                # TODO: watch out for case with listlike value and scalar/empty indexer
-                if is_list_like(value) and is_empty_indexer(indexer, np.array(value)):
-                    return self
+                # TODO: watch out for case with listlike value and scalar indexer
                 dtype, _ = maybe_promote(np.array(value).dtype)
                 return self.astype(dtype).setitem(indexer, value)
 

From a527342b2706a8e23a0af7c41c1ccd94f91dce9e Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 9 Nov 2020 00:15:08 +0100
Subject: [PATCH 04/17] Move test

---
 pandas/tests/frame/indexing/test_setitem.py | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index 128caa387bac7..3e052d91b9252 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -298,13 +298,6 @@ def test_iloc_setitem_bool_indexer(self, klass):
         expected = DataFrame({"flag": ["x", "y", "z"], "value": [2, 3, 4]})
         tm.assert_frame_equal(df, expected)
 
-    def test_setitem_only_false_indexer_dtype_changed(self):
-        # GH: 37550
-        df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
-        df.loc[[False], ["b"]] = 10 - df["c"]
-        expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
-        tm.assert_frame_equal(df, expected)
-
 
 class TestDataFrameSetItemSlicing:
     def test_setitem_slice_position(self):
@@ -346,3 +339,10 @@ def test_setitem_boolean_mask(self, mask_type, float_frame):
         expected = df.copy()
         expected.values[np.array(mask)] = np.nan
         tm.assert_frame_equal(result, expected)
+
+    def test_setitem_only_false_indexer_dtype_changed(self):
+        # GH: 37550
+        df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
+        df.loc[[False], ["b"]] = 10 - df["c"]
+        expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
+        tm.assert_frame_equal(df, expected)

From 345b3c19bfc9c488c6634ba1477cd15619acb43d Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 9 Nov 2020 02:56:52 +0100
Subject: [PATCH 05/17] Adjust test

---
 pandas/tests/frame/indexing/test_setitem.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index 3e052d91b9252..da3cc47fdef14 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -346,3 +346,6 @@ def test_setitem_only_false_indexer_dtype_changed(self):
         df.loc[[False], ["b"]] = 10 - df["c"]
         expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
         tm.assert_frame_equal(df, expected)
+
+        df.loc[[False], ["b"]] = 10 - 1
+        tm.assert_frame_equal(df, expected)

From 3b92d31a4a136be1ebbafce2c5516422a1d59e17 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 14 Nov 2020 13:17:37 +0100
Subject: [PATCH 06/17] Fix whatsnew

---
 doc/source/whatsnew/v1.2.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 3a5e1f16e90fe..b3c070a978bb4 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -469,7 +469,7 @@ Indexing
 - Bug in :meth:`Index.where` incorrectly casting numeric values to strings (:issue:`37591`)
 - Bug in :meth:`Series.loc` and :meth:`DataFrame.loc` raises when numeric label was given for object :class:`Index` although label was in :class:`Index` (:issue:`26491`)
 - Bug in :meth:`DataFrame.loc` returned requested key plus missing values when ``loc`` was applied to single level from :class:`MultiIndex` (:issue:`27104`)
-- Bug in :meth:`DataFrame.loc.__setitem__` changed dtype when indexer was completely ``False`` (:issue:`37550`)
+- Bug in :meth:`DataFrame.loc.__setitem__` changing dtype when indexer was completely ``False`` (:issue:`37550`)
 
 Missing
 ^^^^^^^

From 2f7bf13f778dade9f3dc68bcb1e0d6be4a742aea Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 14 Nov 2020 13:31:36 +0100
Subject: [PATCH 07/17] Make test more clear

---
 pandas/tests/frame/indexing/test_setitem.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index da3cc47fdef14..da0fe84f29c6f 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -340,8 +340,9 @@ def test_setitem_boolean_mask(self, mask_type, float_frame):
         expected.values[np.array(mask)] = np.nan
         tm.assert_frame_equal(result, expected)
 
-    def test_setitem_only_false_indexer_dtype_changed(self):
-        # GH: 37550
+    def test_setitem_loc_only_false_indexer_dtype_changed(self):
+        # GH#37550
+        # Dtype is only changed when value to set is a Series
         df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
         df.loc[[False], ["b"]] = 10 - df["c"]
         expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})

From eb3b204dc1eee203427dd6dc276eebe1ad93f19d Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 15 Nov 2020 19:06:29 +0100
Subject: [PATCH 08/17] Use asarray

---
 pandas/core/internals/blocks.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index a76cf49223280..b9dba6801eaa8 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -928,7 +928,7 @@ def setitem(self, indexer, value):
 
             elif lib.is_scalar(value) and not isna(value):
                 dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
-            elif is_list_like(value) and is_empty_indexer(indexer, np.array(value)):
+            elif is_list_like(value) and is_empty_indexer(indexer, np.asarray(value)):
                 return self
             else:
                 # e.g. we are bool dtype and value is nan

From 9d59de4d070aaa9ef4d65e834763e310fe372a73 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 16 Nov 2020 21:43:09 +0100
Subject: [PATCH 09/17] Parametrize test

---
 pandas/tests/frame/indexing/test_setitem.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index da0fe84f29c6f..b7c45dd320f87 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -340,13 +340,16 @@ def test_setitem_boolean_mask(self, mask_type, float_frame):
         expected.values[np.array(mask)] = np.nan
         tm.assert_frame_equal(result, expected)
 
-    def test_setitem_loc_only_false_indexer_dtype_changed(self):
+    @pytest.mark.parametrize("func", [list, np.array, Series])
+    @pytest.mark.parametrize("value", [[], [False]])
+    def test_setitem_loc_only_false_indexer_dtype_changed(self, func, value):
         # GH#37550
-        # Dtype is only changed when value to set is a Series
+        # Dtype is only changed when value to set is a Series and indexer is
+        # empty/bool all False
         df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
-        df.loc[[False], ["b"]] = 10 - df["c"]
+        df.loc[func(value), ["b"]] = 10 - df["c"]
         expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
         tm.assert_frame_equal(df, expected)
 
-        df.loc[[False], ["b"]] = 10 - 1
+        df.loc[[False], ["b"]] = 9
         tm.assert_frame_equal(df, expected)

From b02e629938478edf290eba74e1814a18fab2a209 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 17 Nov 2020 12:36:12 +0100
Subject: [PATCH 10/17] Move if condition

---
 pandas/core/indexing.py         | 10 +++++-----
 pandas/core/internals/blocks.py |  3 +--
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index c5e331a104726..839f25d91b4cc 100644
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -31,7 +31,7 @@
 from pandas.core.indexers import (
     check_array_indexer,
     is_list_like_indexer,
-    length_of_indexer,
+    length_of_indexer, is_empty_indexer,
 )
 from pandas.core.indexes.api import Index
 
@@ -1683,6 +1683,10 @@ def _setitem_with_indexer_split_path(self, indexer, value):
             # hasattr first, to avoid coercing to ndarray without reason.
             # But we may be relying on the ndarray coercion to check ndim.
             # Why not just convert to an ndarray earlier on if needed?
+            elif lplane_indexer == 0:
+                # We get here in one case via .loc with a all-False mask
+                pass
+
             elif np.ndim(value) == 2:
                 self._setitem_with_indexer_2d_value(indexer, value)
 
@@ -1695,10 +1699,6 @@ def _setitem_with_indexer_split_path(self, indexer, value):
                 # We only get here with len(ilocs) == 1
                 self._setitem_single_column(ilocs[0], value, plane_indexer)
 
-            elif lplane_indexer == 0 and len(value) == len(self.obj.index):
-                # We get here in one case via .loc with a all-False mask
-                pass
-
             else:
                 # per-label values
                 if len(ilocs) != len(value):
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index b9dba6801eaa8..73aef747547e3 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -928,8 +928,7 @@ def setitem(self, indexer, value):
 
             elif lib.is_scalar(value) and not isna(value):
                 dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
-            elif is_list_like(value) and is_empty_indexer(indexer, np.asarray(value)):
-                return self
+
             else:
                 # e.g. we are bool dtype and value is nan
                 # TODO: watch out for case with listlike value and scalar indexer

From 8edc7d0d2db1123d4be1c9d818503a28e5b0d47b Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 17 Nov 2020 12:48:11 +0100
Subject: [PATCH 11/17] Remove import

---
 pandas/core/indexing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 423f6951ec8e1..24983b0b64d00 100644
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -31,7 +31,7 @@
 from pandas.core.indexers import (
     check_array_indexer,
     is_list_like_indexer,
-    length_of_indexer, is_empty_indexer,
+    length_of_indexer,
 )
 from pandas.core.indexes.api import Index
 

From e57e40744c20a0028d729e07904a3c8220080e52 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Wed, 18 Nov 2020 00:06:39 +0100
Subject: [PATCH 12/17] Rename parametrization

---
 pandas/tests/frame/indexing/test_setitem.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index b7c45dd320f87..0311f109126e2 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -340,16 +340,16 @@ def test_setitem_boolean_mask(self, mask_type, float_frame):
         expected.values[np.array(mask)] = np.nan
         tm.assert_frame_equal(result, expected)
 
-    @pytest.mark.parametrize("func", [list, np.array, Series])
+    @pytest.mark.parametrize("box", [list, np.array, Series])
     @pytest.mark.parametrize("value", [[], [False]])
-    def test_setitem_loc_only_false_indexer_dtype_changed(self, func, value):
+    def test_setitem_loc_only_false_indexer_dtype_changed(self, box, value):
         # GH#37550
         # Dtype is only changed when value to set is a Series and indexer is
         # empty/bool all False
         df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
-        df.loc[func(value), ["b"]] = 10 - df["c"]
+        df.loc[box(value), ["b"]] = 10 - df["c"]
         expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
         tm.assert_frame_equal(df, expected)
 
-        df.loc[[False], ["b"]] = 9
+        df.loc[box(value), ["b"]] = 9
         tm.assert_frame_equal(df, expected)

From c14d7b22c723cde7d28ae20815c7881fef3f0cb6 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 21 Dec 2020 21:00:53 +0100
Subject: [PATCH 13/17] Move whatsnew

---
 doc/source/whatsnew/v1.3.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 7671962018144..58587a739e6ec 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -218,7 +218,7 @@ Indexing
 - Bug in :meth:`CategoricalIndex.get_indexer` failing to raise ``InvalidIndexError`` when non-unique (:issue:`38372`)
 - Bug in inserting many new columns into a :class:`DataFrame` causing incorrect subsequent indexing behavior (:issue:`38380`)
 - Bug in :meth:`DataFrame.iloc.__setitem__` and :meth:`DataFrame.loc.__setitem__` with mixed dtypes when setting with a dictionary value (:issue:`38335`)
--
+- Bug in :meth:`DataFrame.loc.__setitem__` changing dtype when indexer was completely ``False`` (:issue:`37550`)
 -
 
 Missing

From 85120c90a18d608c56d4336727ad2705ddfaf9f2 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 18 Apr 2021 00:34:27 +0200
Subject: [PATCH 14/17] Fix dep warning

---
 pandas/tests/frame/indexing/test_setitem.py | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index 38e91a9870d34..cd137c43c50f5 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -841,11 +841,15 @@ def test_setitem_loc_only_false_indexer_dtype_changed(self, box, value):
         # Dtype is only changed when value to set is a Series and indexer is
         # empty/bool all False
         df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
-        df.loc[box(value), ["b"]] = 10 - df["c"]
+        if box == Series and not value:
+            indexer = box(value, dtype="object")
+        else:
+            indexer = box(value)
+        df.loc[indexer, ["b"]] = 10 - df["c"]
         expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
         tm.assert_frame_equal(df, expected)
 
-        df.loc[box(value), ["b"]] = 9
+        df.loc[indexer, ["b"]] = 9
         tm.assert_frame_equal(df, expected)
 
     @pytest.mark.parametrize("indexer", [tm.setitem, tm.loc])

From bae602e70d43b7b7e148384de43cd77c538001c3 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 18 Apr 2021 01:19:47 +0200
Subject: [PATCH 15/17] Fix corner case

---
 pandas/core/indexing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 870010c5bded7..d70f50fa0233a 100644
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -1743,7 +1743,7 @@ def _setitem_with_indexer_split_path(self, indexer, value, name: str):
             if isinstance(value, ABCDataFrame):
                 self._setitem_with_indexer_frame_value(indexer, value, name)
 
-            elif lplane_indexer == 0:
+            elif lplane_indexer == 0 and not isinstance(pi, slice):
                 # We get here in one case via .loc with a all-False mask
                 pass
 

From d334c7ad76306be49ff3d9a76869cd8a984b07c4 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Fri, 10 Dec 2021 15:53:35 +0100
Subject: [PATCH 16/17] Move check

---
 pandas/core/indexing.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 730c2bf0ebcfd..f043a8cee308c 100644
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -1706,10 +1706,6 @@ def _setitem_with_indexer_split_path(self, indexer, value, name: str):
             if isinstance(value, ABCDataFrame):
                 self._setitem_with_indexer_frame_value(indexer, value, name)
 
-            elif lplane_indexer == 0 and not isinstance(pi, slice):
-                # We get here in one case via .loc with a all-False mask
-                pass
-
             elif np.ndim(value) == 2:
                 self._setitem_with_indexer_2d_value(indexer, value)
 
@@ -2062,6 +2058,8 @@ def ravel(i):
             # we have a frame, with multiple indexers on both axes; and a
             # series, so need to broadcast (see GH5206)
             if sum_aligners == self.ndim and all(is_sequence(_) for _ in indexer):
+                if is_empty_indexer(indexer[0], ser._values):
+                    return ser._values.copy()
                 ser = ser.reindex(obj.axes[0][indexer[0]], copy=True)._values
 
                 # single indexer

From f12ef06664c94143bd143780e12f6caa81a505a9 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Fri, 10 Dec 2021 16:01:30 +0100
Subject: [PATCH 17/17] Split tests

---
 doc/source/whatsnew/v1.4.0.rst              |  1 +
 pandas/tests/frame/indexing/test_setitem.py | 21 +++++++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/doc/source/whatsnew/v1.4.0.rst b/doc/source/whatsnew/v1.4.0.rst
index c3cf64c84510e..eb3fcfd74819d 100644
--- a/doc/source/whatsnew/v1.4.0.rst
+++ b/doc/source/whatsnew/v1.4.0.rst
@@ -697,6 +697,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc.__getitem__` incorrectly raising ``KeyError`` when selecting a single column with a boolean key (:issue:`44322`).
 - Bug in setting :meth:`DataFrame.iloc` with a single ``ExtensionDtype`` column and setting 2D values e.g. ``df.iloc[:] = df.values`` incorrectly raising (:issue:`44514`)
 - Bug in indexing on columns with ``loc`` or ``iloc`` using a slice with a negative step with ``ExtensionDtype`` columns incorrectly raising (:issue:`44551`)
+- Bug in :meth:`DataFrame.loc.__setitem__` changing dtype when indexer was completely ``False`` (:issue:`37550`)
 - Bug in :meth:`IntervalIndex.get_indexer_non_unique` returning boolean mask instead of array of integers for a non unique and non monotonic index (:issue:`44084`)
 - Bug in :meth:`IntervalIndex.get_indexer_non_unique` not handling targets of ``dtype`` 'object' with NaNs correctly (:issue:`44482`)
 -
diff --git a/pandas/tests/frame/indexing/test_setitem.py b/pandas/tests/frame/indexing/test_setitem.py
index 65ea67b11864c..9868cec633dff 100644
--- a/pandas/tests/frame/indexing/test_setitem.py
+++ b/pandas/tests/frame/indexing/test_setitem.py
@@ -922,17 +922,26 @@ def test_setitem_boolean_mask(self, mask_type, float_frame):
         expected.values[np.array(mask)] = np.nan
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.xfail(reason="Currently empty indexers are treated as all False")
     @pytest.mark.parametrize("box", [list, np.array, Series])
-    @pytest.mark.parametrize("value", [[], [False]])
-    def test_setitem_loc_only_false_indexer_dtype_changed(self, box, value):
+    def test_setitem_loc_empty_indexer_raises_with_non_empty_value(self, box):
+        # GH#37672
+        df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
+        if box == Series:
+            indexer = box([], dtype="object")
+        else:
+            indexer = box([])
+        msg = "Must have equal len keys and value when setting with an iterable"
+        with pytest.raises(ValueError, match=msg):
+            df.loc[indexer, ["b"]] = [1]
+
+    @pytest.mark.parametrize("box", [list, np.array, Series])
+    def test_setitem_loc_only_false_indexer_dtype_changed(self, box):
         # GH#37550
         # Dtype is only changed when value to set is a Series and indexer is
         # empty/bool all False
         df = DataFrame({"a": ["a"], "b": [1], "c": [1]})
-        if box == Series and not value:
-            indexer = box(value, dtype="object")
-        else:
-            indexer = box(value)
+        indexer = box([False])
         df.loc[indexer, ["b"]] = 10 - df["c"]
         expected = DataFrame({"a": ["a"], "b": [1], "c": [1]})
         tm.assert_frame_equal(df, expected)