[Python-Dev] PEP 567 v2

[Guido van Rossum]

On Thu, Jan 4, 2018 at 3:25 AM, Paul Moore <p.f.moore at gmail.com> wrote:

> On 4 January 2018 at 00:17, Greg Ewing <greg.ewing at canterbury.ac.nz>
> wrote:
> > Guido van Rossum wrote:
> >>
> >> contextvars.copy_context().run(func, <args>)
> >
> > If contexts are immutable, why is there something
> > called copy_context?
>
> Agreed. This was something that bothered me, too. I mentioned it in my
> review, but that seemed to get lost in the other comments in this
> thread...
>
> I get the impression that the logic is that the context is immutable,
> but the ContextVars that it contains aren't, and the copy is deep (at
> least 1 level deep) so you copy then change the value of a ContextVar.
> But rereading that sentence, it sounds confused even to me, so it's
> either not right or the implementation falls foul of "If the
> implementation is hard to explain, it's a bad idea." :-)


It was get_context() in an earlier version of PEP 567. We changed it to
copy_context() believing that that would clarify that you get a clone that
is unaffected by subsequent ContextVar.set() operations (which affect the
*current* context rather than the copy you just got).

[The discussion is fragmentary because Yury is on vacation until the 15th
and I am scrambling for time myself. But your long post is saved, and not
forgotten.]

-- 
--Guido van Rossum (python.org/~guido)
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