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                ## 問題 你需要查找星期中某一天最后出現的日期,比如星期五。 ## 解決方案 Python的datetime模塊中有工具函數和類可以幫助你執行這樣的計算。下面是對類似這樣的問題的一個通用解決方案: #!/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 在交互式解釋器中使用如下: >>> 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) >>> 可選的start_date參數可以由另外一個datetime實例來提供。比如: >>> get_previous_byday('Sunday', datetime(2012, 12, 21)) datetime.datetime(2012, 12, 16, 0, 0) >>> ## 討論 上面的算法原理是這樣的:先將開始日期和目標日期映射到星期數組的位置上(星期一索引為0),然后通過模運算計算出目標日期要經過多少天才能到達開始日期。然后用開始日期減去那個時間差即得到結果日期。 如果你要像這樣執行大量的日期計算的話,你最好安裝第三方包python-dateutil來代替。比如,下面是是使用dateutil模塊中的 `relativedelta()` 函數執行同樣的計算: >>> 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 >>>
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