[Python-Dev] PEP 567 v2

[Guido van Rossum]

Keep MISSING.

On Dec 28, 2017 8:44 AM, "Yury Selivanov" <yselivanov.ml at gmail.com> wrote:

> On Thu, Dec 28, 2017 at 4:51 AM, Victor Stinner
> <victor.stinner at gmail.com> wrote:
> > Hi,
> >
> > I like the new version of the PEP using "read only mapping" and
> > copy_context(). It's easier to understand.
>
> Thanks, Victor!
>
> >
> > I'm ok with seeing a context as a mapping, but I am confused about a
> context
> > variable considered as a mapping item. I still see a context variable as
> a
> > variable, so something which has a value or not. I just propose to rename
> > the default parameter of the ContextVar constructor.
>
> As Nathaniel already explained, a 'default' for ContextVars is
> literally a default -- default value returned when a ContextVar hasn't
> been assigned a value in a context.  So my opinion on this is that
> 'default' is the less ambiguous name here.
>
> [..]
> >
> > * a read-only attribute ``Token.old_value`` set to the value the
> >   variable had before the ``set()`` call, or to ``Token.MISSING``
> >   if the variable wasn't set before.
> >
> >
> > Hum, I also suggest to rename Token.MISSING to Token.NOT_SET. It would be
> > more conistent with the last sentence.
>
> I like MISSING more than NOT_SET, but this is very subjective, of
> course.  If Guido wants to rename it I rename it.
>
>
> > C API
> > -----
> >
> >
> > Would it be possible to make this API private?
>
> We want _decimal and numpy to use the new API, and they will call
> ContextVar.get() on basically all operations, so it needs to be as
> fast as possible.  asyncio/uvloop also want the fastest copy_context()
> and Context.run() possible, as they use them for *every* callback.  So
> I think it's OK for us to add new C APIs here.
>
>
> >
> > 2. ``int PyContextVar_Get(PyContextVar *, PyObject *default_value,
> > PyObject **value)``:
> >     (...)  ``value`` is always a borrowed
> >    reference.
> >
> >
> > I'm not sure that it's a good idea to add a new public C function which
> > returns a borrowed reference. I would prefer to only use (regular) strong
> > references in the public API.
>
> Sure, I'll change it.
>
> Yury
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