BUG: AssertionError on Series.append(DataFrame) fix #30975 (#31036)

Author: hvardhan20

pandas/core/series.py

@@ -2475,7 +2475,7 @@ def searchsorted(self, value, side="left", sorter=None):
     # -------------------------------------------------------------------
     # Combination
 
-    def append(self, to_append, ignore_index=False, verify_integrity=False) -> "Series":
+    def append(self, to_append, ignore_index=False, verify_integrity=False):
         """
         Concatenate two or more Series.
 
@@ -2552,10 +2552,8 @@ def append(self, to_append, ignore_index=False, verify_integrity=False) -> "Seri
             to_concat.extend(to_append)
         else:
             to_concat = [self, to_append]
-        return self._ensure_type(
-            concat(
-                to_concat, ignore_index=ignore_index, verify_integrity=verify_integrity
-            )
+        return concat(
+            to_concat, ignore_index=ignore_index, verify_integrity=verify_integrity
         )
 
     def _binop(self, other, func, level=None, fill_value=None):

pandas/tests/series/methods/test_append.py

@@ -61,6 +61,16 @@ def test_append_tuples(self):
 
         tm.assert_series_equal(expected, result)
 
+    def test_append_dataframe_regression(self):
+        # GH 30975
+        df = pd.DataFrame({"A": [1, 2]})
+        result = df.A.append([df])
+        expected = pd.DataFrame(
+            {0: [1.0, 2.0, None, None], "A": [None, None, 1.0, 2.0]}, index=[0, 1, 0, 1]
+        )
+
+        tm.assert_frame_equal(expected, result)
+
 
 class TestSeriesAppendWithDatetimeIndex:
     def test_append(self):