[Python-Dev] PEP 550 v4 (original) (raw)

Kevin Conway kevinjacobconway at gmail.com
Wed Aug 30 13:39:50 EDT 2017

Can Execution Context be implemented outside of CPython

I know I'm well late to the game and a bit dense, but where in the pep is the justification for this assertion? I ask because we buy something to solve the same problem in Twisted some time ago: https://bitbucket.org/hipchat/txlocal . We were able to leverage generator/coroutine decorators to preserve state without modifying the runtime.

Given that this problem only exists in runtime that multiplex coroutines on a single thread and the fact that coroutine execution engines only exist in user space, why doesn't it make more sense to leave this to a library that engines like asyncio and Twisted are responsible for standardising on?

On Wed, Aug 30, 2017, 09:40 Yury Selivanov <yselivanov.ml at gmail.com> wrote:

On Wed, Aug 30, 2017 at 9:44 AM, Yury Selivanov <yselivanov.ml at gmail.com> wrote: [..] >> FYI, I've been sketching an alternative solution that addresses these kinds >> of things. I've been hesitant to post about it, partly because of the >> PEP550-based workarounds that Nick, Nathaniel, Yury etc. have been >> describing, and partly because that might be a major distraction from other >> useful discussions, especially because I wasn't completely sure yet about >> whether my approach has some fatal flaw compared to PEP 550 ;). > > We'll never know until you post it. Go ahead.

The only alternative design that I considered for PEP 550 and ultimately rejected was to have a the following thread-specific mapping: { var1: [stack of values for var1], var2: [stack of values for var2] } So the idea is that when we set a value for the variable in some frame, we push it to its stack. When the frame is done, we pop it. This is a classic approach (called Shallow Binding) to implement dynamic scope. The fatal flow that made me to reject this approach was the CM protocol (enter). Specifically, context managers need to be able to control values in outer frames, and this is where this approach becomes super messy. Yury

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