'Return the zone to use for listing resources.
:param  zone: A name, zone object, None, or \'all\'
:type   zone: ``str`` or :class:`GCEZone` or ``None``
:return:  A zone object or None if all zones should be considered
:rtype:   :class:`GCEZone` or ``None``'

def _set_zone(self, zone):

    zone = (zone or self.zone)
    if ((zone == 'all') or (zone is None)):
        return None
    if (not hasattr(zone, 'name')):
        zone = self.ex_get_zone(zone)
    return zone
