BUG: Fix incosistent behavior of 2d-indexed loc-set to Series #59933

pandas/core/indexing.py

@@ -2371,12 +2371,15 @@ 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 all(is_sequence(_) or isinstance(_, slice) for _ in indexer):
                 ser_values = ser.reindex(obj.axes[0][indexer[0]])._values
 
                 # single indexer
                 if len(indexer) > 1 and not multiindex_indexer:
-                    len_indexer = len(indexer[1])
+                    if isinstance(indexer[1], slice):
+                        len_indexer = len(obj.loc[indexer[1]].axes[1])
+                    else:
+                        len_indexer = len(indexer[1])
                     ser_values = (
                         np.tile(ser_values, len_indexer).reshape(len_indexer, -1).T
                     )

pandas/tests/indexing/test_loc.py

@@ -3297,3 +3297,43 @@ def test_loc_reindexing_of_empty_index(self):
         df.loc[Series([False] * 4, index=df.index, name=0), 0] = df[0]
         expected = DataFrame(index=[1, 1, 2, 2], data=["1", "1", "2", "2"])
         tm.assert_frame_equal(df, expected)
+
+    def test_loc_set_series_to_multiple_columns(self):
+        # GH 59933
+        df = DataFrame([[1, 2, 3], [4, 5, 6]], columns=list("ABC"))
+        df.loc[0:3, ["A", "B", "C"]] = Series([10, 20, 30])
+        expected = DataFrame([[10, 10, 10], [20, 20, 20]], columns=list("ABC"))
+        tm.assert_frame_equal(df, expected)
+
+        df = DataFrame([[1, 2, 3], [4, 5, 6]], columns=list("ABC"))
+        df.loc[:, ["A", "B", "C"]] = Series([10, 20, 30])
+        expected = DataFrame([[10, 10, 10], [20, 20, 20]], columns=list("ABC"))
+        tm.assert_frame_equal(df, expected)
+
+        df = DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]], columns=list("ABC"))
+        df.loc[df["A"] > 0, ["A", "B", "C"]] = Series([10, 20, 30])
+        expected = DataFrame(
+            [[10, 10, 10], [20, 20, 20], [30, 30, 30]], columns=list("ABC")
+        )
+        tm.assert_frame_equal(df, expected)
+
+        df = DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]], columns=list("ABC"))
+        df.loc[:, ["A", "B", "C"]] = Series([10, 20, 30])
+        expected = DataFrame(
+            [[10, 10, 10], [20, 20, 20], [30, 30, 30]], columns=list("ABC")
+        )
+        tm.assert_frame_equal(df, expected)
+
+        df = DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]], columns=list("ABC"))
+        df.loc[0:4, ["A", "B", "C"]] = Series([10, 20, 30])
+        expected = DataFrame(
+            [[10, 10, 10], [20, 20, 20], [30, 30, 30]], columns=list("ABC")
+        )
+        tm.assert_frame_equal(df, expected)
+
+        df = DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]], columns=list("ABC"))
+        df.loc[:, "A":"C"] = Series([10, 20, 30])
+        expected = DataFrame(
+            [[10, 10, 10], [20, 20, 20], [30, 30, 30]], columns=list("ABC")
+        )
+        tm.assert_frame_equal(df, expected)