def utc_to_utcf(utc):
    """
    Converts a UTC datetime into UTC string format %Y-%m-%dT%H:%M:%SZ.

    :param utc: python UTC datetime
    :return: a string in UTC string format %Y-%m-%dT%H:%M:%SZ
    """
    return "%04d-%02d-%02dT%02d:%02d:%02dZ" % (utc.year, utc.month, utc.day, utc.hour, utc.minute, utc.second)