def generate_random_string(size=6, chars=string.ascii_uppercase + string.digits):
    """Generate random string.

    :param size: Length of the returned string. Default is 6.
    :param chars: List of the usable characters. Default is string.ascii_uppercase + string.digits.
    :type size: int
    :type chars: str
    :return: The random string.
    :rtype: str
    """
    return ''.join(random.choice(chars) for _ in range(size))