'Get a chunk from the IFF file'

def __getitem__(self, id_):

    assert isinstance(id_, text_type)
    if (not is_valid_chunk_id(id_)):
        raise KeyError('AIFF    key    must    be    four    ASCII    characters.')
    try:
        return self.__chunks[id_]
    except KeyError:
        raise KeyError(('%r    has    no    %r    chunk' % (self.__fileobj, id_)))
