download mp4s

def download_mp4(from_idx, to_idx, _params):
    """
    download mp4s
    """
    succ = set()
    fail = set()
    for idx in range(from_idx, to_idx):
        name = 's' + str(idx)
        save_folder = '{src_path}/{nm}'.format(src_path=_params['src_path'], nm=name)
        if idx == 0 or os.path.isdir(save_folder):
            continue
        script = "http://spandh.dcs.shef.ac.uk/gridcorpus/{nm}/video/{nm}.mpg_vcd.zip".format( \
                    nm=name)
        down_sc = 'cd {src_path} && curl {script} --output {nm}.mpg_vcd.zip && \
                    unzip {nm}.mpg_vcd.zip'.format(script=script,
                                                   nm=name,
                                                   src_path=_params['src_path'])
        try:
            print(down_sc)
            os.system(down_sc)
            succ.add(idx)
        except OSError as error:
            print(error)
            fail.add(idx)
    return (succ, fail)