Will "hello" always be printed?

[MRAB]

On 2022-10-08 00:40, Cameron Simpson wrote:
> On 07Oct2022 20:16, Robin van der veer <robinvdveer at gmail.com> wrote:
>>If I have two processes communicating through a JoinableQueue, and I do the
>>following:
>>
>>process 1:
>>
>>    queue.put(1) #unfished tasks = 1
>>    queue.join() #block until unfished tasks = 0
>>    print('hello')[/python]
>>
>>process 2:
>>
>>    queue.get()
>>    queue.task_done() #unfished tasks = 0
>>    queue.put(1) #unfinished tasks 1[/python]
>>the unfished tasks refers to what is written in the documentation (
>>https://docs.python.org/3/library/multiprocessing.html#multiprocessing.JoinableQueue.join
>>)
>>
>>will 'hello' always be printed? Or is there a chance that the put in
>>process 2 executes before process 1 noticed that it should unblock?
> 
> I had to read this closely. Yes, the second `put(1)` could execute
> before the `join()` commences (or tests), and the `hello` would be
> blocked still.
> 
>>It seems that the whole point of join() is that 'hello' should always be
>>printed, but I just want to make sure that I understand it correctly.
> 
> That's the purpose of using `join`, but you need to use it correctly.
> The "some tasks are not completed" condition which `join` supports
> doesn't fit what you're doing.
> 
> So yes, you're correct in your concern.
> 
> Maybe 2 queues would suit you better? Maybe not if they are common.
> 
I would go with 2 queues: 1 for input and 1 for output. The outputted 
item would be either a result or an indication of an error.

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