datetime - How to calculate difference between two dates in weeks in python

Question

I'm trying to calculate the difference between two dates in "weeks of year". I can get the datetime object and get the days etc but not week numbers. I can't, of course, subtract dates because weekends can't be ensured with that.

I tried getting the week number using d1.isocalendar()[1] and subtracting d2.isocalendar()[1] but the issue is that isocalendar()[1] returns December 31, 2012 as week 1 (which supposedly is correct) but that means my logic cannot span over this date.

For reference, here's my complete code:

def week_no(self):
    ents = self.course.courselogentry_set.all().order_by('lecture_date')
    l_no = 1
    for e in ents:
        if l_no == 1: 
             starting_week_of_year = e.lecture_date.isocalendar()[1] # get week of year
             initial_year = e.lecture_date.year   
        if e == self: 
            this_year = e.lecture_date.year
            offset_week = (this_year - initial_year) * 52
            w_no = e.lecture_date.isocalendar()[1] - starting_week_of_year + 1 + offset_week
            break 
        l_no += 1
    return w_no  

With this code, the lecture on Dec 31, 2012 ends up being -35.

Answer 1

How about calculating the difference in weeks between the Mondays within weeks of respective dates? In the following code, monday1 is the Monday on or before d1 (the same week):

from datetime import datetime, timedelta

monday1 = (d1 - timedelta(days=d1.weekday()))
monday2 = (d2 - timedelta(days=d2.weekday()))

print 'Weeks:', (monday2 - monday1).days / 7

Returns 0 if both dates fall withing one week, 1 if on two consecutive weeks, etc.