code: def normalize_column_names(df):
      r
      columns = df.columns if hasattr(df, 'columns') else df
      columns = [c.lower().replace(' ', '_') for c in columns]
      return columns
docs: r""" Clean up whitespace in column names. See better version at `pugnlp.clean_columns`
  
      >>> df = pd.DataFrame([[1, 2], [3, 4]], columns=['Hello World', 'not here'])
      >>> normalize_column_names(df)
      ['hello_world', 'not_here']