> I found the weird code in the example in several projects. I have corrected it to use the pool as a context manager or to call close(), but this means that users are doing this and it used to work and not it does not: technically is a regression.

That's why I'm asking for a revert :-) I suggest to revert this change immediately from 2.7, 3.6 and 3.7, and later see what can be done for master.

Even if we design carefully an API, there will be always someone to misuse it :-) I would prefer to stop promoting such bad code and find a transition to more correct code.

I disagree that a child should keep its parent alive.

I would be ok to use a *weak reference* from the child to the parent to detect when the parent goes away, and maybe trigger an action in that case. For example, raise an exception or log a warning.