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

Ethan Furman ethan at stoneleaf.us
Wed Oct 18 14:10:46 EDT 2017

On 10/18/2017 10:50 AM, Yury Selivanov wrote:
> 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.

I don't see why this is a concern -- Python is a "consenting adults" language.  If users decide to start mucking around 
with advanced behavior and something breaks, well, they own all the pieces! ;)

Unless it's extremely difficult to not seg-fault in such a situation I don't think this is a valid argument.


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