千千惠生活达人注册你需要查找星期中某一天最后出现的日期，比如星期五。
Python的datetime模块中有工具函数和类可以帮助你执行这样的计算。下面是对类似这样的问题的一个通用解决方案：
#!/usr/bin/env python
#-*-encoding: utf-8-*-“””
Topic:最后的周五Desc:
“””
from datetime import datetime, timedelta
weekdays=[‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,
‘Friday’,’Saturday’,’Sunday’]
def get_previous_byday(dayname, start_date=None):
if start_date is None:
start_date= datetime.today() day_num= start_date.weekday()
day_num_target= weekdays.index(dayname)
days_ago=(7+ day_num-day_num_target)% 7 if days_ago== 0:
days_ago= 7
target_date= start_date-timedelta(days=days_ago) return target_date
在交互式解释器中使用如下：
>>> datetime.today()# For reference
datetime.datetime(2012, 8, 28, 22, 4, 30, 263076)>>> get_previous_byday(‘Monday’)
datetime.datetime(2012, 8, 27, 22, 3, 57, 29045)
>>> get_previous_byday(‘Tuesday’)# Previous week, not today datetime.datetime(2012, 8, 21, 22, 4, 12, 629771)
>>> get_previous_byday(‘Friday’)
datetime.datetime(2012, 8, 24, 22, 5, 9, 911393)>>>
可选的start_date参数可以由另外一个datetime实例来提供。比如：>>> get_previous_byday(‘Sunday’, datetime(2012, 12, 21)) datetime.datetime(2012, 12, 16, 0, 0)
>>>
讨论
上面的算法原理是这样的：先将开始日期和目标日期映射到星期数组的位置上(星期一索引为0)，然后通过模运算计算出目标日期要经过多少天才能到达开始日期。然后用开始日期减去那个时间差即得到结果日期。
如果你要像这样执行大量的日期计算的话，你最好安装第三方包python-dateutil来代替。比如，下面是是使用dateutil模块中的relativedelta()函数执行同样的计算：
>>> from datetime import datetime
>> from dateutil.relativedelta import relativedelta>>>
from dateutil.rrule import*>>> d= datetime.now()>>> print(d)
2012-12-2316:31:52.718111>>># Next Friday
>>> print(d+ relativedelta(weekday=FR)) 2012-12-2816:31:52.718111
>>>
>>># Last Friday
>>> print(d+ relativedelta(weekday=FR(-1))) 2012-12-2116:31:52.718111
>>>
