I guess I found a bug in the documented Queue.join() example [1].

The problem is the break condition for the while loop of the worker. If the item is None, the loop breaks, but the worker never calls item.task_done(). Thus the q.join() statement never returns, because the last task (None) remains unfinished.

This should solve the issue:

```
if item is None:
  item.task_done()
  break
```

[1] https://docs.python.org/3/library/queue.html#queue.Queue.join

Sorry, I missed that q.join() is executed *before* the "None" item is added to the queue.

In my real-world case I called q.join() *after* I added the "None" item.