同时，关于datetime也是简单介绍。因为有很多东西需要自己去使用，去查帮助才最有效。
例子：
计算上一个星期五并输出。
解答：
import datetime, calendar
lastFriday = datetime.date.today( )
oneday = datetime.timedelta(days=1)
lastFriday -= oneday
while lastFriday.weekday( ) != calendar.FRIDAY:
lastFriday -= oneday
print lastFriday.strftime('%A, %d-%b-%Y')
输出结果：
Friday, 09-May-2008
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