code: def test_fast_unique_flag_from_arrow() -> None:
      df = pl.DataFrame(
          {
              "colB": ["1", "2", "3", "4", "5", "5", "5", "5"],
          }
      ).with_columns([pl.col("colB").cast(pl.Categorical)])
  
      filtered = df.to_arrow().filter(
          [True, False, True, True, False, True, True, True]
      )
      assert (
          pl.from_arrow(filtered)
          .select(pl.col("colB").n_unique())
          .item()
          == 4
      )
  
imports: from __future__ import annotations
  import contextlib
  import io
  import operator
  from typing import TYPE_CHECKING, Any, Callable, Literal
  import pytest
  import polars
  from polars import StringCache
  from polars.exceptions import CategoricalRemappingWarning, StringCacheMismatchError
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 32 for calling a global function with example usage: test_fast_unique_flag_from_arrow() without return types