def fill_subparser(subparser):
    """Sets up a subparser to convert YouTube audio files.

    Adds the compulsory `--youtube-id` flag as well as the optional
    `sample` and `channels` flags.

    Parameters
    ----------
    subparser : :class:`argparse.ArgumentParser`
        Subparser handling the `youtube_audio` command.

    """
    subparser.add_argument(
        '--youtube-id', type=str, required=True,
        help=("The YouTube ID of the video from which to extract audio, "
              "usually an 11-character string.")
    )
    subparser.add_argument(
        '--channels', type=int, default=1,
        help=("The number of audio channels to convert to. The default of 1"
              "means audio is converted to mono.")
    )
    subparser.add_argument(
        '--sample', type=int, default=16000,
        help=("The sampling rate in Hz. The default of 16000 is "
              "significantly downsampled compared to normal WAVE files; "
              "pass 44100 for the usual sampling rate.")
    )
    return convert_youtube_audio
Sets up a subparser to convert YouTube audio files.

    Adds the compulsory `--youtube-id` flag as well as the optional
    `sample` and `channels` flags.

    Parameters
    ----------
    subparser : :class:`argparse.ArgumentParser`
        Subparser handling the `youtube_audio` command.