[Python-Dev] PEP 525, third round, better finalization (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Sat Sep 3 21:22:47 EDT 2016

Nick Coghlan wrote:

For synchronous code, that's a relatively easy burden to push back onto the programmer - assuming fair thread scheduling, a with statement can ensure reliably ensure prompt resource cleanup.

That assurance goes out the window as soon as you explicitly pause code execution inside the body of the with statement - it doesn't matter whether its via yield, yield from, or await, you've completely lost that assurance of immediacy.

I don't see how this is any worse than a thread containing an ordinary with-statement that waits for something that will never happen. If that's the case, then you've got a deadlock, and you have more to worry about than resources not being released.

I think what all this means is that an event loop must not simply drop async tasks on the floor. If it's asked to cancel a task, it should do that by throwing an appropriate exception into it and letting it unwind itself.

To go along with that, the programmer needs to understand that he can't just fire off a task and abandon it if it uses external resources and is not guaranteed to finish under its own steam. He needs to arrange a timeout or other mechanism to cancel it if it doesn't complete in a timely manner.

If those things are done, an async with should be exactly as adequate for resource cleanup as an ordinary with is in a thread. It also shouldn't be necessary to have any special protocol for finalising an async generator; async with together with a way of throwing an exception into a task should be all that's needed.

-- Greg

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