def _concat_csv(csv_file_list, output_file):
    """
    Helper function for concat_csv
    """
    import pandas as pd
    input_dfs = list(map(lambda x: pd.read_csv(x, header=[0], index_col=[0]
        ), csv_file_list))
    result_df = pd.concat(input_dfs, axis=0, sort=False)
    result_df.to_csv(output_file)
    return result_df
