日付操作.py

import datetime
import calendar


def lastday_of_month(date):
    y = date.year
    m = date.month
    last_d = calendar.monthrange(y, m)[1]
    return date.replace(day=last_d)


def next_month(date):
    delta = datetime.timedelta(days=1)
    return lastday_of_month(date) + delta


def prev_month(date):
    delta = datetime.timedelta(days=1)
    first_day_of_month = date.replace(day=1)
    prev_last_month = first_day_of_month - delta
    return prev_last_month.replace(day=1)

# import datetime as dt
# today = dt.date.today()
# print "Today: " + today.strftime('%Y-%m-%d')
# one_day = dt.timedelta(days=1)
# print today+one_day
# print today-one_day
# print today-one_day*31 #.replace(days=1)
# pre_month = today-one_day*31 #.replace(days=1)
# pre_month = pre_month.replace(day=1)
# print pre_month

http://success.tracpath.com/blog/2009/07/09/python%E3%81%A7%E3%81%AE%E6%97%A5%E4%BB%98%E9%96%A2%E9%80%A3%E5%87%A6%E7%90%86/

calender ライブラリのmonthrangeがつかえそう。