Gets an instance implementing type <_type>

def _get_a(self, _type):
        """ Gets an instance implementing type <_type> """
        tmp = self._get_all(_type)
        ret = pick(tmp)
        if len(tmp) != 1:
            self.l.warn(("get_a: %s all implement %s; " +
                         "picking %s") % (tmp, _type, ret))
        return ret