def get_first_tarfile_member(tar_file):
    """
    Returns a file like object of the first member of the given tarfile.

    :param tar_file: The tarfile object to get the first member of
    :type tar_file: tarfile.TarFile

    :return: A file like object containing the data of the first member in the given tarfile

    :raise AssertionError: If the given tar file does not contain members
    """
    member = tar_file.next()
    if member is None:
        raise AssertionError('Given tarfile does not contain a member')
    return tar_file.extractfile(member)
