How to use the priority.DeadlockError function in priority

def check_next_stream_consistent(self):
        """
        If we ask priority for the next stream, it always returns a sensible
        result.
        """
        try:
            next_stream_id = next(self.tree)
        except priority.DeadlockError:
            assert self.blocked_stream_ids ^ {0} == self.stream_ids
        else:
            stream = self.tree._streams[next_stream_id]

            # If a stream is selected, then it isn't blocked
            assert stream.active

            # If a stream is selected, then its parent is either the root
            # stream or blocked
            parent = stream.parent
            assert (parent.stream_id == 0) or (not parent.active)

def test_priority_tree_raises_deadlock_error_if_all_blocked(self):
        """
        Assuming all streams are blocked and none can progress, asking for the
        one with the next highest priority fires a DeadlockError.
        """
        tree = readme_tree()
        for stream_id in range(1, 12, 2):
            tree.block(stream_id)

        with pytest.raises(priority.DeadlockError):
            next(tree)

async def send_task(self) -> None:
        # This should be run in a seperate task to the rest of this
        # class. This allows it seperately choose when to send,
        # crucially in what order.
        while not self.closed:
            try:
                stream_id = next(self.priority)
            except priority.DeadlockError:
                await self.has_data.wait()
                await self.has_data.clear()
            else:
                await self._send_data(stream_id)

connection is used with maximal efficiency.

        This function will execute if data is available: if all data is
        exhausted, the function will place a deferred onto the L{H2Connection}
        object and wait until it is called to resume executing.
        """
        # If producing has stopped, we're done. Don't reschedule ourselves
        if not self._stillProducing:
            return

        stream = None

        while stream is None:
            try:
                stream = next(self.priority)
            except priority.DeadlockError:
                # All streams are currently blocked or not progressing. Wait
                # until a new one becomes available.
                assert self._sendingDeferred is None
                self._sendingDeferred = Deferred()
                self._sendingDeferred.addCallback(self._sendPrioritisedData)
                return

        # Wait behind the transport.
        if self._consumerBlocked is not None:
            self._consumerBlocked.addCallback(self._sendPrioritisedData)
            return

        self.resetTimeout()

        remainingWindow = self.conn.local_flow_control_window(stream)
        frameData = self._outboundStreamQueues[stream].popleft()

connection is used with maximal efficiency.

        This function will execute if data is available: if all data is
        exhausted, the function will place a deferred onto the L{H2Connection}
        object and wait until it is called to resume executing.
        """
        # If producing has stopped, we're done. Don't reschedule ourselves
        if not self._stillProducing:
            return

        stream = None

        while stream is None:
            try:
                stream = next(self.priority)
            except priority.DeadlockError:
                # All streams are currently blocked or not progressing. Wait
                # until a new one becomes available.
                assert self._sendingDeferred is None
                self._sendingDeferred = Deferred()
                self._sendingDeferred.addCallback(self._sendPrioritisedData)
                return

        # Wait behind the transport.
        if self._consumerBlocked is not None:
            self._consumerBlocked.addCallback(self._sendPrioritisedData)
            return

        remainingWindow = self.conn.local_flow_control_window(stream)
        frameData = self._outboundStreamQueues[stream].popleft()
        maxFrameSize = min(self.conn.max_outbound_frame_size, remainingWindow)

connection is used with maximal efficiency.

        This function will execute if data is available: if all data is
        exhausted, the function will place a deferred onto the L{H2Connection}
        object and wait until it is called to resume executing.
        """
        # If producing has stopped, we're done. Don't reschedule ourselves
        if not self._stillProducing:
            return

        stream = None

        while stream is None:
            try:
                stream = next(self.priority)
            except priority.DeadlockError:
                # All streams are currently blocked or not progressing. Wait
                # until a new one becomes available.
                assert self._sendingDeferred is None
                self._sendingDeferred = Deferred()
                self._sendingDeferred.addCallback(self._sendPrioritisedData)
                return

        # Wait behind the transport.
        if self._consumerBlocked is not None:
            self._consumerBlocked.addCallback(self._sendPrioritisedData)
            return

        remainingWindow = self.conn.local_flow_control_window(stream)
        frameData = self._outboundStreamQueues[stream].popleft()
        maxFrameSize = min(self.conn.max_outbound_frame_size, remainingWindow)