code: def test_fail_column_name_in_columns(missingdata_df):
      """Checks that the output `column_name` is not already in df
      columns.
      """
      with pytest.raises(ValueError):
          missingdata_df.flag_nulls(column_name="a")
  
file_path: pyjanitor/tests/functions/test_flag_nulls.py
imports: import pandas
  import pytest
  from pandas.testing import assert_frame_equal
  from janitor.functions import flag_nulls
label_desc: Use image node_id 6 for calling a global function with example usage: test_fail_column_name_in_columns(missingdata_df) without return types