class YearMixin

def _get_current_year(self, date):
    """Return the start date of the current interval."""
    return date.replace(month=1, day=1)

def _get_next_year(self, date):
    """
    Return the start date of the next interval.
    The interval is defined by start date <= item date < next start date.
    """
    try:
        return date.replace(year=date.year + 1, month=1, day=1)
    except ValueError:
        raise Http404(_("Date out of range"))

def get_next_year(self, date):
    """Get the next valid year."""
    return _get_next_prev(self, date, is_previous=False, period="year")

def get_previous_year(self, date):
    """Get the previous valid year."""
    return _get_next_prev(self, date, is_previous=True, period="year")

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

def get_year_format(self):
    """
    Get a year format string in strptime syntax to be used to parse the
    year from url variables.
    """
    return self.year_format