Python: Friday 13th

#!/usr/bin/env python
# @hieuza [at] gmail.com
# 04.Jan.2014

# the months with 13th Friday of a year
import datetime

months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
days = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
days_leap = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

# start_day: the weekday of 1st January.
# 0: Monday,... 6: Sunday
def friday13_month(days, start_day):
    accum = [13]
    for x in days[:-1]:
        accum.append(accum[-1] + x)

    # weekday of day 13, if 01/Jan is on Monday (0)
    # must minus 1, because day started at 1
    day13_start0 = [(x - 1) % 7 for x in accum]

    # day 13, in considerate of the start day
    day13 = [(x + start_day) % 7 for x in day13_start0]

    # which month has Friday 13
    res = []
    for i, x in enumerate(day13):
        if x == 4: # Friday
            res.append(i)

    return res

def is_leap_year(year):
    return year % 400 == 0 or (year % 100 != 0 and year % 4 == 0)

def which_month_friday13(year):
    day1 = datetime.date(year, 1, 1)
    start_day = day1.weekday()

    # print 'year:', year, 'start_day:', start_day

    if is_leap_year(year):
        res = friday13_month(days_leap, start_day)
    else:
        res = friday13_month(days, start_day)

    print [months[i] for i in res]

    return res

def test_is_leap_year():
    print '2000:', is_leap_year(2000)
    print '1800:', is_leap_year(1800)
    print '2008:', is_leap_year(2008)

def test_friday13(year):
    # year = 2014
    res = which_month_friday13(year)

    # check the correct Friday 13
    for m in res:
        d = datetime.date(year, m + 1, 13)
        if d.weekday() != 4:
            print 'ERROR: 13/%s is not Friday' % months[m]
    # check that there's no other Friday 13
    for m in xrange(12):
        if m in res: continue
        d = datetime.date(year, m + 1, 13)
        if d.weekday() == 4:
            print 'ERROR: 13th %s is Friday' % months[m]

def test_interactive():
    print 'Enter a year to print the month with Friday 13. 0 to exit'

    while True:
        year = int(raw_input('year: '))
        if year == 0:
            break

        test_friday13(year)

if __name__ == '__main__':
    # test_is_leap_year()
    # test_friday13(2014)
    test_interactive()