Spotify Problem - Best Before www.spotify.com/se/jobs/tech/best-before/, https://github.com/ocelma/spotify-puzzles (test cases)

bestBefore.py

#python3
import itertools, datetime

def readFile(path):
    """ Requires only one arguement, the path of the file to be read """

    with open(path) as file:            #auto closes file after
        content = file.readline()

    return content

def set_up(date):

    """ Format Date into a lsit of three integers """
    date = str(date)
    date = date.split('/')

    values = []

    for i in date:
        values.append(int(i))

    return values

def solution(date):

    """ Main Logic to solve problem """

    values = set_up(date)

    #Fix the case where one of the integers is 0 (2000)
    for i in values:

        if i == 0:
            values.remove(i)
            values.append(2000)
        #Numbers that can never be used for anything but the year
        if i > 31 and i < 100:
            values.remove(i)
            values.append(2000 + i)

    #Generate all possible dates
    perms = list(itertools.permutations(values))
    dates = []

    #Converting combinations of dates into datetime objects enables
    #easy testing for invalid dates, which are ignored (by handling the exception created)
    #allows sorting to find the earliest date

    for i in perms:
        try:
            if i[0] < 31 : dates.append(datetime.date(2000 + i[0],i[1],i[2]))
            else: dates.append(datetime.date(i[0],i[1],i[2]))
        except:
            continue

    dates.sort()

    if dates == []:
        return (str(date) + ' is illegal')
    else:
        return dates[0]


#Run Program

#If you have a file location, Use the commented path below
#date = readFile(path)
#print(solution(date))

#If you have the exact date, Use the commented path below
#print(solution('02/4/67'))
#print(solution('31/9/73'))

date = input('Please input the date:')
print(solution(date))