def digits_increase(num):
    """boolean test for number whose digits increase from left to right.

    Arg:
        num: the number to test. expecting a six-digit number.

    Returns:
        boolean
    """
    output = True
    digits = [int(i) for i in str(num)]
    for i, dig in enumerate(digits[:-1]):
        if dig > digits[i + 1]:
            output = False
    return output