fix bug which prevented .apply to being called with function which returned list

Author: Marco Gorelli

doc/source/whatsnew/v1.0.1.rst

@@ -15,7 +15,7 @@ including other versions of pandas.
 
 Bug fixes
 ~~~~~~~~~
-
+- bug ``Groupby.apply`` was returning ``TypeError`` if called with TypeError if called with `lambda x: x.index.to_list()` (:issue:`31441`)
 
 Categorical
 ^^^^^^^^^^^

pandas/_libs/reduction.pyx

@@ -1,4 +1,4 @@
-from copy import copy
+from copy import copy, deepcopy
 from distutils.version import LooseVersion
 
 from cython import Py_ssize_t
@@ -499,7 +499,9 @@ def apply_frame_axis0(object frame, object f, object names,
                 # `piece` might not have an index, could be e.g. an int
                 pass
 
-            if not is_scalar(piece):
+            if isinstance(piece, list):
+                piece = deepcopy(piece)
+            elif not is_scalar(piece):
                 # Need to copy data to avoid appending references
                 if hasattr(piece, "copy"):
                     piece = piece.copy(deep="all")

pandas/tests/groupby/test_apply.py

@@ -827,3 +827,10 @@ def test_apply_index_has_complex_internals(index):
     df = DataFrame({"group": [1, 1, 2], "value": [0, 1, 0]}, index=index)
     result = df.groupby("group").apply(lambda x: x)
     tm.assert_frame_equal(result, df)
+
+
+def test_apply_function_returns_list():
+    df = pd.DataFrame(["A", "A", "B", "B"], columns=["groups"])
+    result = df.groupby("groups").apply(lambda x: x.index.to_list())
+    expected = pd.Series([[0, 1], [2, 3]], index=pd.Index(["A", "B"], name="groups"))
+    tm.assert_series_equal(result, expected)