[Python-Dev] PEP 567 v2

Guido van Rossum guido at python.org
Tue Jan 2 12:57:36 EST 2018

Oh, the "Specification" section in the PEP is too brief on several of these
subjects. It doesn't really specify what var.get() does if the value is not
set, nor does it even mention var.get(<default>) except in the code
examples for var.reset(). It's also subtle that ctx[var] returns the
default (if there is one). I suppose it will raise if there isn't one --
resulting in the somewhat surprising behavior where `var in ctx` may be
true but `ctx[var]` may raise. And what does it raise? (All these questions
are answered by the code, but they should be clearly stated in the PEP.)

I would really like to invite more people to review this PEP! I expect I'll
be accepting it in the next two weeks, but it needs to go through more
rigorous review.

On Thu, Dec 28, 2017 at 4:48 PM, Victor Stinner <victor.stinner at gmail.com>

> NLe 28 déc. 2017 11:20 AM, "Nathaniel Smith" <njs at pobox.com> a écrit :
> On Thu, Dec 28, 2017 at 1:51 AM, Victor Stinner
> <victor.stinner at gmail.com> wrote:
> > var = ContextVar('var', default=42)
> >
> > and:
> >
> > var = ContextVar('var')
> > var.set (42)
> >
> > behaves the same, no?
> No, they're different. The second sets the value in the current
> context. The first sets the value in all contexts that currently
> exist, and all empty contexts created in the future.
> Oh, that's an important information. In this case, "default" is the best
> name.
> The PEP may be more explicit about the effect on all contexts. Proposition
> of documentation:
> "The optional *default* parameter is the default value in all contexts. If the
> variable is not set in the current context, it is returned by by
> context[var_name] and by var.get(), when get() is called without the
> default parameter."
> Victor
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--Guido van Rossum (python.org/~guido)
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