def num_pad(num, length):
    """
    Returns given number with zero's at prefix to match given length
    :param num: int
    :param length: int
    :return: str
    """

    num_str = str(num)
    length_str = str(length)
    num_chars = len(num_str)
    length_chars = len(length_str)
    if num_chars < length_chars:
        diff = length_chars - num_chars
        return '{}{}'.format('0' * diff, num_str)

    return num_str