code: def test_approx_n_unique() -> None:
      df1 = pl.DataFrame({"a": [None, 1, 2], "b": [None, 2, 1]})
  
      assert_frame_equal(
          df1.select(pl.approx_n_unique("b")),
          pl.DataFrame({"b": pl.Series(values=[3], dtype=pl.UInt32)}),
      )
  
      assert_frame_equal(
          df1.select(pl.approx_n_unique(pl.col("b"))),
          pl.DataFrame({"b": pl.Series(values=[3], dtype=pl.UInt32)}),
      )
  
      assert_frame_equal(
          df1.select(pl.col("b").approx_n_unique()),
          pl.DataFrame({"b": pl.Series(values=[3], dtype=pl.UInt32)}),
      )
  
imports: from __future__ import annotations
  from datetime import timedelta
  from typing import TYPE_CHECKING
  import numpy
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 15 for calling a global function with example usage: test_approx_n_unique() without return types