code: def test_num_rows() -> None:
      df = pl.DataFrame({"a": [1, 2], "b": [3, 4]})
      dfi = PolarsDataFrame(df)
      assert dfi.num_rows() == 2
  
imports: from __future__ import annotations
  import pytest
  import polars
  from polars.interchange.dataframe import PolarsDataFrame
  from polars.interchange.protocol import CopyNotAllowedError
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 5 for calling a global function with example usage: test_num_rows() without return types