code: def test_n_unique() -> None:
      s = pl.Series("s", [11, 11, 11, 22, 22, 33, None, None, None])
      assert s.n_unique() == 4
  
imports: from __future__ import annotations
  import pytest
  import polars
label_desc: Use image node_id 1 for calling a global function with example usage: test_n_unique() without return types