Returns tube by its name

            :param name: Tube name
            :returns: ``self.tube_cls`` instance
                (by default :class:`asynctnt_queue.Tube`)

def tube(self, name):
        """
            Returns tube by its name

            :param name: Tube name
            :returns: ``self.tube_cls`` instance
                (by default :class:`asynctnt_queue.Tube`)
        """
        if name in self._tubes:
            return self._tubes[name]

        assert name, 'Tube name must be specified'
        t = self._tube_cls(self, name)
        self._tubes[name] = t
        return t