BUG: DataFrame.loc not aligning rhs df for single block case

doc/source/whatsnew/v1.5.0.rst

@@ -882,6 +882,7 @@ Indexing
 - Bug in :meth:`Series.__setitem__` when setting ``boolean`` dtype values containing ``NA`` incorrectly raising instead of casting to ``boolean`` dtype (:issue:`45462`)
 - Bug in :meth:`Series.__setitem__` where setting :attr:`NA` into a numeric-dtype :class:`Series` would incorrectly upcast to object-dtype rather than treating the value as ``np.nan`` (:issue:`44199`)
 - Bug in :meth:`DataFrame.loc` when setting values to a column and right hand side is a dictionary (:issue:`47216`)
+- Bug in :meth:`DataFrame.loc` when setting a :class:`DataFrame` not aligning index in some cases (:issue:`47578`)
 - Bug in :meth:`Series.__setitem__` with ``datetime64[ns]`` dtype, an all-``False`` boolean mask, and an incompatible value incorrectly casting to ``object`` instead of retaining ``datetime64[ns]`` dtype (:issue:`45967`)
 - Bug in :meth:`Index.__getitem__`  raising ``ValueError`` when indexer is from boolean dtype with ``NA`` (:issue:`45806`)
 - Bug in :meth:`Series.__setitem__` losing precision when enlarging :class:`Series` with scalar (:issue:`32346`)

pandas/core/frame.py

@@ -4706,10 +4706,11 @@ def _sanitize_column(self, value) -> ArrayLike:
         """
         self._ensure_valid_index(value)
 
-        # We should never get here with DataFrame value
-        if isinstance(value, Series):
+        # We can get there through isetitem with a DataFrame
+        # or through loc single_block_path
+        if isinstance(value, DataFrame):
             return _reindex_for_setitem(value, self.index)
-        elif isinstance(value, dict):
+        elif is_dict_like(value):
             return _reindex_for_setitem(Series(value), self.index)
 
         if is_list_like(value):

pandas/tests/frame/indexing/test_indexing.py

@@ -1298,6 +1298,15 @@ def test_loc_expand_empty_frame_keep_midx_names(self):
         )
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize("val", ["x", 1])
+    @pytest.mark.parametrize("idxr", ["a", ["a"]])
+    def test_loc_setitem_rhs_frame(self, idxr, val):
+        # GH#47578
+        df = DataFrame({"a": [1, 2]})
+        df.loc[:, "a"] = DataFrame({"a": [val, 11]}, index=[1, 2])
+        expected = DataFrame({"a": [np.nan, val]})
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameIndexingUInt64:
     def test_setitem(self, uint64_frame):

pandas/tests/frame/indexing/test_setitem.py

@@ -721,6 +721,17 @@ def test_setitem_aligning_dict_with_index(self, vals):
         )
         tm.assert_frame_equal(df, expected)
 
+    def test_setitem_rhs_dataframe(self):
+        # GH#47578
+        df = DataFrame({"a": [1, 2]})
+        df["a"] = DataFrame({"a": [10, 11]}, index=[1, 2])
+        expected = DataFrame({"a": [np.nan, 10]})
+        tm.assert_frame_equal(df, expected)
+
+        df = DataFrame({"a": [1, 2]})
+        df.isetitem(0, DataFrame({"a": [10, 11]}, index=[1, 2]))
+        tm.assert_frame_equal(df, expected)
+
 
 class TestSetitemTZAwareValues:
     @pytest.fixture