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Mon Aug 28 11:50:04 EDT 2017 · https://mail.python.org/pipermail/python-dev/2017-August/149009.html

On Sun, Aug 27, 2017 at 4:01 PM, Nathaniel Smith <njs at pobox.com> wrote:
> I believe that the current status is:
>
> - assigning None isn't treated specially – it does mask any underlying
> values (which I think is what we want)

Correct.

>
> - there is currently no way to "unmask"
>
> - but it's generally agreed that there should be a way to do that, at least
> in some cases, to handle the save/restore issue I raised. It's just that
> Yury & Elvis wanted to deal with restructuring the PEP first before doing
> more work on the api details.

Yes. I think it's a good time to start a discussion about this, I can
list a couple ideas here.

For checking if a context variable has a value in the topmost LC, we
can add two new keyword arguments to the "ContextVar.lookup()" method:

   ContextVar.lookup(*, default=None, topmost=False)

If `topmost` is set to `True`, `lookup` will only check the topmost LC.

For deleting a value from the topmost LC we can add a new
"ContextVar.delete()" method.

Yury

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[Python-Dev] Pep 550 and None/masking