[Python-Dev] Timeout for PEP 550 / Execution Context discussion

[Yury Selivanov]

On Wed, Oct 18, 2017 at 1:06 PM, Guido van Rossum <guido at python.org> wrote:
> On Tue, Oct 17, 2017 at 9:40 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
[..]
>> By contrast, "contextvars.set_ctx" would need various wrappers to handle
>> correctly reverting the context change, and would hence be prone to "changed
>> the active context without changing it back" bugs (which can be especially
>> fun when you're dealing with a shared pool of worker threads or processes).
>
>
> So let's have both.

The main reason why I don't like 'set_ctx()' is because it would make
it harder for us to adopt PEP 550-like design later in the future
(*if* we need that.)

PEP 550 is designed in such a way, that 'generator.send()' is the only
thing that can control the actual stack of LCs.  If users can call
'set_ctx' themselves, it means that it's no longer safe for
'generator.send()' to simply pop the topmost LC from the stack.  This
can be worked around, potentially, but the we don't actually need
'set_ctx' in asyncio or in any other async framework.  There is simply
no hard motivation to have it.  That's why I'd like to have just
Context.run(), because it's sufficient, and it doesn't burn the bridge
to PEP 550-like design.

Yury