Paul Ganssle <p.gans...@gmail.com> added the comment:
I don't really know what Python was doing in version 2.3, and I don't have immediate access to a Python 2.3 interpreter, but at least for %U and %W, datetime is calling the platform's `strftime` under the hood, so presumably if this is a bug it's a bug in glibc and the other providers of `strftime`. Digging a bit more, %U and %W appear to be the the same for all Sundays if (and only if) the year starts on a Monday: import calendar from datetime import datetime from dateutil import rrule rr = rrule.rrule(freq=rrule.WEEKLY, byweekday=rrule.SU, dtstart=datetime(1900, 1, 1), until=datetime(2100, 1, 1)) for dt in rr: is_same = dt.strftime("%U") == dt.strftime("%W") year_starts_monday = calendar.weekday(dt.year, 1, 1) == 0 assert is_same == year_starts_monday This seems to be the right behavior, because %U and %W count all days before their respective "first day of the week" as "week 0", and week 1 starts with the relevant day of the week. If the year starts with Monday, week 1 is 1 January - 7 January according to %W (year starts on Monday), and week 1 is 7 January - 13 January according to %U (year starts on Sunday), thus all Sundays will be in the same "week number" in both systems. > %U is supposed to work with the week numbering system common (as I understand > it) in North America, where (according to Wikipedia) week 1 begins on a > Sunday, and contains both 1 January and the first Saturday of the year. While > I am not familiar with that system, Excel 2016 is, and it reports The documentation for %U says: > Week number of the year (Sunday as the first day of the week) as a zero > padded decimal number. All days in a new year preceding the first Sunday are > considered to be in week 0. This means that week 1 would only contain the first Saturday of the month and January 1st on years that start on Sunday. The Python documentation is consistent with the man page for strftime(3): http://man7.org/linux/man-pages/man3/strftime.3.html ---------- versions: +Python 3.6, Python 3.7, Python 3.8 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue35535> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com