python - 查找任何开始日的一周开始时间的更简单方法

Question

给定这两个变量，我们需要找到一周的开始(日期时间):

1. now - 当前日期时间
2. START_DAY - 我们认为“一周的开始”的星期几(0-6，其中 0 是星期一，6 是星期日)

我们可以使用循环，但这对于一个简单的问题来说似乎步骤太多了:

week_start = datetime(now.year, now.month, now.day)
while week_start.weekday() != START_DAY:
    week_start = week_start - timedelta(days=1)

如果 START_DAY 始终为 0(星期一)，这真的很容易:

week_start = datetime(now.year, now.month, now.day) - timedelta(days=now.weekday())

或者总是6(周日):

week_start = datetime(now.year, now.month, now.day) - timedelta(days=(0 if now.weekday() == START_DAY else now.weekday() + 1))

计算任意 START_DAY 值的一周开始时间的简单方法是什么？

Answer 1

START_DAY 和 now 之间的天数为 now.weekday() - START_DAY。但是，如果今天早于 START_DAY(例如，如果今天是星期三，而 START_DATE 是 6 = 星期日)，则该数字将为负数，从今天中减去它会得到 future 的日期。

因为您可能不希望这样，所以您需要将差值模 7:

week_start = now.date() - timedelta(days=(now.weekday() - START_DAY) % 7)

示例:

>>> from datetime import datetime, timedelta
>>> now = datetime(2017, 2, 1)
>>> print(now.strftime("%d/%m/%y (%a)"))
01/02/17 (Wed)
>>> for START_DAY in range(7):
...     start_week = now.date() - timedelta(days=(now.weekday() - START_DAY) % 7)
...     print(start_week.strftime("%d/%m/%y (%a)"))
... 
30/01/17 (Mon)
31/01/17 (Tue)
01/02/17 (Wed)
26/01/17 (Thu)
27/01/17 (Fri)
28/01/17 (Sat)
29/01/17 (Sun)