Python 按月增加datetime月份的问题

#coding=utf-8

import datetime

# input datetime1, and an month offset
# return the result datetime

def datetime_offset_by_month(datetime1, n = 1):

    # create a shortcut object for one day
    one_day = datetime.timedelta(days = 1)

    # first use div and mod to determine year cycle
    q,r = divmod(datetime1.month + n, 12)

    # create a datetime2
    # to be the last day of the target month
    datetime2 = datetime.datetime(
        datetime1.year + q, r + 1, 1) - one_day

'''
   if input date is the last day of this month
   then the output date should also be the last
   day of the target month, although the day
   www.iplaypy.com
   may be different.
   for example:
   datetime1 = 8.31
   datetime2 = 9.30
'''

    if datetime1.month != (datetime1 + one_day).month:
        return datetime2

'''
    if datetime1 day is bigger than last day of
    target month, then, use datetime2
    for example:
    datetime1 = 10.31
    datetime2 = 11.30
'''

    if datetime1.day >= datetime2.day:
        return datetime2

'''
 then, here, we just replace datetime2's day
 with the same of datetime1, that's ok.
'''

    return datetime2.replace(day = datetime1.day)



#------------玩蛇网-----www.iplaypy.com---------------

d1 = datetime.datetime(2008, 8, 17)
d2 = datetime_offset_by_month(d1, 13)

print '2008-8-17 + 13 month'
print d2
print



d1 = datetime.datetime(2008, 8, 31)
d2 = datetime_offset_by_month(d1, 13)

print '2008-8-31 + 13 month'
print d2
print



d1 = datetime.datetime(2007, 1, 30)
d2 = datetime_offset_by_month(d1, 13)

print '2007-1-30 + 13 month'
print d2
print

result = """
2008-8-17 + 13 month
2009-09-17 00:00:00

2008-8-31 + 13 month
2009-09-30 00:00:00

2007-1-30 + 13 month
2008-02-29 00:00:00
"""