[Python-Dev] PEP 567 v2 (original) (raw)

Paul Moore p.f.moore at gmail.com
Fri Jan 5 05:41:01 EST 2018

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 getcontext() in an earlier version of PEP 567. We changed it to > copycontext() 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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