constructing Datetime objects with ISO week numbers

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Tuesday, November 4. 2008

constructing Datetime objects with ISO week numbers

an old problem, but almost no library implements it, so here is my shot at it in python:

import datetime

def DateTimeFromIsoWeek(year, isoweek=1):
       """DateTimeFromIsoWeek(year,isoweek=1)

Returns a DateTime instance pointing to the given ISO week/year
           The weekday defaults to 4, which corresponds to Thursday in the
           ISO numbering. The time part is set to 00:00:00.
       """
       # anyone knows what assertions could be done for the year?
     assert 0 < isoweek < 54
     # start stupidly with 1st Januray of the given year:
     d = datetime.datetime(year, 1, 1, 0, 0, 0)
     # what weekday is this?
     dow = d.isocalendar()[2]
     # advance to next thursday, if not already on a thursday:
     if dow < 4: # if we are in monday - wednesday:
        d += datetime.timedelta(7 - dow - 3)
     elif dow > 4: # if we are in friday - sunday
        d += datetime.timedelta(7 - dow + 4)
     # advance to the given week:
     d += datetime.timedelta((isoweek-1) * 7)
     if isoweek == 53: # this might be a user error, not all years have 53 weeks, so check it
        y = d.isocalendar()[0]
     if y != year: # we ended in a different year -> the given year only has 52 weeks!
        raise Exception, "The year '%i' does not have an ISO week '%i'" % (year, isoweek)

return d

Posted by Pol at 22:30 | Comments (0) | Trackbacks (0)

Defined tags for this entry: code, date, python, time

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