code: def test_list_join() -> None:
      df = pl.DataFrame(
          {
              "a": [["ab", "c", "d"], ["e", "f"], ["g"], [], None],
              "separator": ["&", None, "*", "_", "*"],
          }
      )
      out = df.select(pl.col("a").list.join("-"))
      assert out.to_dict(as_series=False) == {
          "a": ["ab-c-d", "e-f", "g", "", None]
      }
      out = df.select(pl.col("a").list.join(pl.col("separator")))
      assert out.to_dict(as_series=False) == {
          "a": ["ab&c&d", None, "g", "", None]
      }
  
imports: from __future__ import annotations
  from datetime import date, datetime
  import numpy
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 4 for calling a global function with example usage: test_list_join() without return types