find earliest possible legal date in A/B/C format

earliest_date.py

#!/usr/bin/python3

import sys
import itertools
from datetime import datetime


def main(s):
    " find earliest possible legal date or raise ValueError "

    def check_str(s):
        if not s.isdigit():
            raise ValueError('invalid value %s' % s)
        if len(s) not in (1, 2, 4):
            raise ValueError('invalid value %s' % s)
        return s

    def check_number(n):
        if not ((0 <= n <= 99) or (2000 <= n <= 2999)):
            raise ValueError('invalid value %d' % n)
        return n

    res = [check_number(int(check_str(i))) for i in s.split('/')]

    try:
        (a, b, c) = res
    except ValueError:
        raise ValueError('invalid date, has to be 3 items')

    valid_dates = []

    def normalize_year(n):
        if n <= 99:
            n += 2000
        return n

    for (year, month, day) in itertools.permutations((a, b, c)):

        year = normalize_year(year)

        try:
            valid_dates.append(datetime(year, month, day))
        except ValueError:
            pass

    if not valid_dates:
        raise ValueError('invalid date')

    return min(valid_dates).strftime('%Y-%m-%d')

if __name__ == '__main__':

    if len(sys.argv) < 2:
        print('Usage: {} FILE'.format(__file__))
        sys.exit(1)

    with open(sys.argv[1]) as fp:
        s = fp.read().strip()

    try:
        print(main(s))
    except ValueError:
        print(repr(s), 'is illegal')
        sys.exit(1)