code: def test_convert_col_names_to_str():
      df = pd.DataFrame(np.random.random((10, 2)))
      assert all(not isinstance(col, str) for col in df.columns)
      df = _convert_col_names_to_str(df)
      assert all(isinstance(col, str) for col in df.columns)
  
imports: import numpy
  import pandas
  import pytest
  from unittest import TestCase, SkipTest
  from hvplot.util import check_crs, is_list_like, process_crs, process_xarray, _convert_col_names_to_str, instantiate_crs_str
label_desc: Use image node_id 10 for calling a global function with example usage: test_convert_col_names_to_str() without return types