'Determine which zones need to be enabled or disabled on the ELB'

def _set_zones(self):

    if self.zones:
        if self.purge_zones:
            zones_to_disable = list((set(self.elb.availability_zones) - set(self.zones)))
            zones_to_enable = list((set(self.zones) - set(self.elb.availability_zones)))
        else:
            zones_to_disable = None
            zones_to_enable = list((set(self.zones) - set(self.elb.availability_zones)))
        if zones_to_enable:
            self._enable_zones(zones_to_enable)
        if zones_to_disable:
            self._disable_zones(zones_to_disable)
