code: def test_fast_explode_flag() -> None:
      df1 = pl.DataFrame({"values": [[[1, 2]]]})
      assert df1.clone().vstack(df1)["values"].flags["FAST_EXPLODE"]
  
      # test take that produces a null in list
      df = pl.DataFrame({"a": [1, 2, 1, 3]})
      df_b = pl.DataFrame(
          {"a": [1, 2], "c": [["1", "2", "c"], ["1", "2", "c"]]}
      )
      assert df_b["c"].flags["FAST_EXPLODE"]
  
      # join produces a null
      assert not (df.join(df_b, on=["a"], how="left").select(["c"]))[
          "c"
      ].flags["FAST_EXPLODE"]
  
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 14 for calling a global function with example usage: test_fast_explode_flag() without return types