class WeekMixin

def _get_current_week(self, date):
    """
    Return the start date of the current interval.
    """
    return date - datetime.timedelta(self._get_weekday(date))

def _get_next_week(self, date):
    """
    Return the start date of the next interval.
    The interval is defined by start date <= item date < next start date.
    """
    return date + datetime.timedelta(days=7 - self._get_weekday(date))

def _get_weekday(self, date):
    """
    Return the weekday for a given date.
    The first day according to the week format is 0 and the last day is 6.
    """
    week_format = self.get_week_format()
    if week_format == '%W':                 # week starts on Monday
        return date.weekday()
    elif week_format == '%U':               # week starts on Sunday
        return (date.weekday() + 1) % 7
    else:
        raise ValueError("unknown week format: %s" % week_format)

def get_next_week(self, date):
    """
    Get the next valid week.
    """
    return _get_next_prev(self, date, is_previous=False, period='week')

def get_previous_week(self, date):
    """
    Get the previous valid week.
    """
    return _get_next_prev(self, date, is_previous=True, period='week')

def get_week(self):
    """
    Return the week for which this view should display data
    """
    week = self.week
    if week is None:
        try:
            week = self.kwargs['week']
        except KeyError:
            try:
                week = self.request.GET['week']
            except KeyError:
                raise Http404(_("No week specified"))
    return week

def get_week_format(self):
    """
    Get a week format string in strptime syntax to be used to parse the
    week from url variables.
    """
    return self.week_format