gistfile1.py

from trees.models import Tree, Post, Member
from datetime import datetime, timedelta
def percentRetained():
    '''Iterate over periods of 2 weeks, filter out all users who didn't add a post during that 2-weeks-period.
    Start with particular group of two weeks, see who used it for that 2-week period.
    For every period of two weeks, filter out people who are not'''

    today = datetime.today()
    experiment_duration = 7 * 4 * 3
    retained_members = None
    for d in range(0, experiment_duration, 7 * 2):
        date_range = (today, today + timedelta(d))
        members_this_period = [p.tree.owner for p in Post.objects.filter(
                created__range=date_range).all()]
        if retained_members == None:
            members_on_starting_period = members_this_period
            retained_members = members_this_period
            pass
        new_retained_members = []
        for member in retained_members:
            if member in members_this_period:
                new_retained_members.append(member)
        retained_members = new_retained_members
    print 'There were %s members on the starting period' % (len(members_on_starting_period))
    print 'There were %s retained members after %s days' % (len(retained_members), experiment_duration)
    return float(len(retained_members)) / float(len(members_on_starting_period))