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Fri Jan 5 05:41:01 EST 2018 · https://mail.python.org/pipermail/python-dev/2018-January/151612.html

On 4 January 2018 at 23:58, Guido van Rossum <guido at python.org> wrote:
> On Thu, Jan 4, 2018 at 9:27 AM, Paul Moore <p.f.moore at gmail.com> wrote:
>>
>> On 4 January 2018 at 15:56, Guido van Rossum <guido at python.org> wrote:
>> > 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).
>>
>> Ah thanks. In which case, simply changing the emphasis to avoid the
>> implication that Context objects are immutable (while that may be true
>> in a technical/implementation sense, it's not really true in a design
>> sense if ContextVar.set modifies the value of a variable in a context)
>> is probably sufficient.
>
>
> Do you have a specific proposal for a wording change? PEP 567 describes
> Context as "a read-only mapping, implemented using an immutable dictionary."
> This sounds all right to me -- "read-only" is weaker than "immutable". Maybe
> the implementation should not be mentioned here? (The crux here is that a
> given Context acts as a variable referencing an immutable dict -- but it may
> reference different immutable dicts at different times.)

I've been struggling to think of good alternative wordings (it's a
case of "I'm not sure what you're trying to say, so I can't work out
how you should say it", unfortunately). The best I can come up with is

"""
A Context is a mapping from ContextVar objects to their values. The
Context itself exposes the Mapping interface, so cannot be modified
directly - to modify the value associated with a variable you need to
use the ContextVar.set() method.
"""

Does that explain things correctly? One thing I am sure of is that we
should remove "implemented using an immutable dictionary" - it's an
implementation detail, and adds nothing but confusion to mention it
here.

Paul

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[Python-Dev] PEP 567 v2