BUG: Fix bug in loc setitem changing the dtype when condition is False

doc/source/whatsnew/v1.2.0.rst

@@ -465,6 +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`)
 
 Missing
 ^^^^^^^

pandas/core/internals/blocks.py

@@ -920,10 +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)
-
             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)
 

pandas/tests/frame/indexing/test_setitem.py

@@ -298,6 +298,13 @@ 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):