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

[Yury Selivanov]

On Tue, Oct 17, 2017 at 2:25 PM, Guido van Rossum <guido at python.org> wrote:
> On Tue, Oct 17, 2017 at 8:54 AM, Yury Selivanov <yselivanov.ml at gmail.com>
[..]
>> My way of thinking about this: "get_execution_context()" returns you a
>> shallow copy of the current EC (at least conceptually).  So making any
>> modifications on it won't affect the current environment.  The only
>> way to actually apply the modified EC object to the environment will
>> be its 'run(callable)' method.
>
>
> I understand that you don't want to throw away the implementation work
> you've already done. But I find that the abstractions you've introduced are
> getting in the way of helping people understand what they can do with
> context variables, and I really want to go back to a model that is *much*
> closer to understanding how instance variables are just self.__dict__. (Even
> though there are possible complications due to __slots__ and @property.)

I don't really care about the implementation work that has already
been done, it's OK if I write it from scratch again.

I actually like what you did in
https://github.com/gvanrossum/pep550/blob/master/simpler.py, it seems
reasonable.  The only thing that I'd change is to remove "set_ctx"
from the public API and add "Context.run(callable)".  This makes the
API more flexible to potential future changes and amendments.


> In short, I really don't think there's a need for context variables to be
> faster than instance variables.

Well, even our current idea about the API, doesn't really prohibit us
from adding a cache to ContextVar.get().  That would be an
implementation detail, right?  Same as our latest optimization for
creating bound methods (CALL_METHOD & LOAD_METHOD opcodes), which
avoids creating bound method instances when it's OK not to.


Yury