How to calculate difference between two dates in weeks in python

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.

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.

You may want to refer the Python CookBook (2005 edition) Recipe 3.3. The following code snippet is from the cookbook, does what you require.

from dateutil import rrule
import datetime
def weeks_between(start_date, end_date):
    weeks = rrule.rrule(rrule.WEEKLY, dtstart=start_date, until=end_date)
    return weeks.count()