[Python-Dev] PEP 567 pre v3

Guido van Rossum guido at python.org
Fri Jan 12 14:38:55 EST 2018

I think we've debated the design of ContextVar and default values enough.
Regardless of the philosophical questions around "what is a variable", I
agree that Yury's current design is right, so let's move on to other topics
(if there are any).

On Thu, Jan 11, 2018 at 12:55 AM, Paul Moore <p.f.moore at gmail.com> wrote:

> On 11 January 2018 at 07:39, Chris Jerdonek <chris.jerdonek at gmail.com>
> wrote:
> > But to your question, like it or not, I think the API encourages this
> > way of thinking because the get() method is on the ContextVar itself,
> > and so it's the ContextVar which is doing the looking up rather than
> > just fulfilling the role of a key name. The API brings to mind other
> > containers and things holding values like dict.get(), queue.get(),
> > BytesIO.getvalue(), and container type's object.__getitem__(), etc. So
> > I think one will need to be prepared for many or most users having
> > this conception with the current API. (I think renaming to something
> > like ContextVar.lookup() or even ContextVar.value() would go a long
> > way towards dispelling that, but Guido said earlier in the thread that
> > he likes the shorter name.)
> I can only offer anecdotal evidence, but I am perfectly comfortable
> with seeing ContextVars as names (variables) that refer to values, and
> not as containers of values. The "Var" part of the class name is what
> makes that link in my head, I think, and it is a stronger association
> for me than the idea that get() implies a container.
> So I'm 100% fine, personally, with ContextVars as names that refer to
> values (that you access via the get() method), and with the Context as
> a hidden lookup table for those values (corresponding to
> globals()/locals()). I'm also OK on that same basis with ContextVars
> having an "unset" state, and with it being unusual/difficult to get
> back to the unset state once you've set a value.
> tl;dr If you think of a ContextVar as a "variable" or "name", the
> current design makes sense (at least to me).
> Paul
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--Guido van Rossum (python.org/~guido)
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