wip

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/extension/conftest.py

@@ -126,6 +126,7 @@ def box_in_series(request):
         lambda x: [1] * len(x),
         lambda x: Series([1] * len(x)),
         lambda x: x,
+        lambda x: x.index.to_list()
     ],
     ids=["scalar", "list", "series", "object"],
 )