REGR: fix DataFrame.agg case with list-like return value (pandas-dev#29632)

Author: jorisvandenbossche

pandas/core/base.py

@@ -602,9 +602,9 @@ def _aggregate_multiple_funcs(self, arg, _level, _axis):
         if not len(results):
             raise ValueError("no results")
 
-        if all(np.ndim(x) > 0 for x in results):
+        try:
             return concat(results, keys=keys, axis=1, sort=False)
-        else:
+        except TypeError:
 
             # we are concatting non-NDFrame objects,
             # e.g. a list of scalars

pandas/tests/frame/test_apply.py

@@ -1259,6 +1259,23 @@ def test_non_callable_aggregates(self):
 
         assert result == expected
 
+    def test_agg_listlike_result(self):
+        # GH-29587 user defined function returning list-likes
+        df = DataFrame(
+            {"A": [2, 2, 3], "B": [1.5, np.nan, 1.5], "C": ["foo", None, "bar"]}
+        )
+
+        def func(group_col):
+            return list(group_col.dropna().unique())
+
+        result = df.agg(func)
+        expected = pd.Series([[2, 3], [1.5], ["foo", "bar"]], index=["A", "B", "C"])
+        tm.assert_series_equal(result, expected)
+
+        result = df.agg([func])
+        expected = expected.to_frame("func").T
+        tm.assert_frame_equal(result, expected)
+
     @pytest.mark.parametrize(
         "df, func, expected",
         chain(