Get date from ISO week number in Python

Question

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What’s the best way to find the inverse of datetime.isocalendar()?

I have an ISO 8601 year and week number, and I need to translate this to the date of the first day in that week (Monday). How can I do this?

datetime.strptime() takes both a %W and a %U directive, but neither adheres to the ISO 8601 weekday rules that datetime.isocalendar() use.

Update: Python 3.6 supports the %G, %V and %u directives also present in libc, allowing this one-liner:

>>> from datetime import datetime
>>> datetime.strptime('2011 22 1', '%G %V %u')
datetime.datetime(2011, 5, 30, 0, 0)

Update 2: Python 3.8 added the fromisocalendar() method, which is even more intuitive:

>>> from datetime import datetime
>>> datetime.fromisocalendar(2011, 22, 1)
datetime.datetime(2011, 5, 30, 0, 0)

Asked By: Erik Cederstrand

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Answer 1

%W takes the first Monday to be in week 1 but ISO defines week 1 to contain 4 January. So the result from

datetime.strptime('2011221', '%Y%W%w')

is off by one iff the first Monday and 4 January are in different weeks.
The latter is the case if 4 January is a Friday, Saturday or Sunday.
So the following should work:

from datetime import datetime, timedelta, date
def tofirstdayinisoweek(year, week):
    ret = datetime.strptime('%04d-%02d-1' % (year, week), '%Y-%W-%w')
    if date(year, 1, 4).isoweekday() > 4:
        ret -= timedelta(days=7)
    return ret

Answered By: Uwe Kleine-König

Answer 2

With the isoweek module you can do it with:

from isoweek import Week
d = Week(2011, 40).monday()

Answered By: gisle

Get date from ISO week number in Python

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