## 问题 你需要查找星期中某一天最后出现的日期，比如星期五。 ## 解决方案 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) 2012-12-23 16:31:52.718111 >>> # Next Friday >>> print(d + relativedelta(weekday=FR)) 2012-12-28 16:31:52.718111 >>> >>> # Last Friday >>> print(d + relativedelta(weekday=FR(-1))) 2012-12-21 16:31:52.718111 >>>