从星期数获取星期开始日期

Question

This question is related to Get date from week number , and is possibly a duplicate of the latter, however, I think what is suggested in the accepted answer to that question does not really work.

In [6]: datetime.datetime.strptime('2019-18-1', "%Y-%W-%w")
Out[6]: datetime.datetime(2019, 5, 6, 0, 0)

Notice how it returns Monday 2019-5-6. However, according to the calendar (I use http://whatweekisit.org for reference), 2019-5-6 the Monday of week 19.

Similarly, the example provided in the original question:

In [7]: datetime.datetime.strptime('2013-26-1', "%Y-%W-%w")
Out[7]: datetime.datetime(2013, 7, 1, 0, 0)

According to http://whatweekisit.org/calendar-2013.html 2013-7-1 is the Monday of week 27.

Also

In [8]: datetime.datetime.strptime('2019-18-1', "%Y-%W-%w").isocalendar()[1]
Out[8]: 19

Notice how I give week 18 to strptime , and get week 19 back from isocalendar .

I am completely lost and would very much appreciate if someone could explain what is going on here. My original goal though is to get week start date from week number.

Answer 1

Based off of my testing, datetime does not consider the first week of 2019 (ie Jan 1-Jan 6) as week 1 because it isn't a full week; December 31st, 2018 is part of the week but is not in 2019. I suppose you'll have to accomodate for that by checking the output of datetime.datetime.strptime('year-1-1', "%Y-%W-%w") == datetime.datetime.strptime('year-0-1', "%Y-%W-%w") . If false, subtract 1.

2018 is an example of a year where datetime does return the same value as isocalendar because the first Monday of the year is Jan 1.

Answer 2

From the isocalendar docs :

The ISO year consists of 52 or 53 full weeks, and where a week starts on a Monday and ends on a Sunday. The first week of an ISO year is the first (Gregorian) calendar week of a year containing a Thursday. This is called week number 1, and the ISO year of that Thursday is the same as its Gregorian year.

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On the other hand, strptime starts from the first full week, in fact, since 2019 starts from Tuesday, they start from different weeks:

import datetime as dt
strp_first = dt.datetime.strptime('2019-1-1', "%Y-%W-%w")

>>> print(strp_first)
2019-01-07 00:00:00

>>> print(strp_first.isocalendar()[1])
2

While in 2021, which starts from Friday:

strp_first = dt.datetime.strptime('2021-1-1', "%Y-%W-%w")

>>> print(strp_first)
2021-01-04 00:00:00

>>> print(strp_first.isocalendar()[1])
1