code: def test_list_mean() -> None:
      assert pl.DataFrame(
          {"a": [[1], [1, 2, 3], [1, 2, 3, 4], [1, 2, 3, 4, 5]]}
      ).select(pl.col("a").list.mean()).to_dict(as_series=False) == {
          "a": [1.0, 2.0, 2.5, 3.0]
      }
  
      assert pl.DataFrame(
          {"a": [[1], [1, 2, 3], [1, 2, 3, 4], None]}
      ).select(pl.col("a").list.mean()).to_dict(as_series=False) == {
          "a": [1.0, 2.0, 2.5, None]
      }
  
imports: from __future__ import annotations
  import pickle
  from datetime import date, datetime, time
  from typing import TYPE_CHECKING, Any
  import pandas
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 20 for calling a global function with example usage: test_list_mean() without return types