[Python-Dev] PEP 567 pre v3

[Nick Coghlan]

On 11 January 2018 at 10:44, Nathaniel Smith <njs at pobox.com> wrote:
> It may have gotten lost in that email, but my actual favorite approach
> is that we make the signatures:
>
> ContextVar(name, *, initial_value)  # or even (*, name, initial_value)
> ContextVar.get()
> ContextVar.set(value)
>
> so that when you create a ContextVar you always state the initial
> value, whatever makes sense in a particular case. (Obviously None will
> be a very popular choice, but this way it won't be implicit, and
> no-one will be surprised to see it returned from get().)

I also like this idea, and I think it aligns better with "dict.get"
than it may first appear: the trick is that in this simplified API
"ContextVar.get()" would be akin to
"get_thread_state().current_context.get(cv, cv.initial_value)" (and
PEP 568 wouldn't really change that).

So both the key and the default value are specified at ContextVar
initialisation time, and if you want to supply a non-standard default,
then you need to break the ContextVar abstraction, and access
"contextvars.copy_context().get(cv, custom_default)" instead.

If we later decide that we do want to support raising an exception for
unitialised access after all, then we could introduce a "missing"
callback to the ContextVar constructor that would be akin to
defaultdict's "default_factory" callback (with the requirement
becoming that you have to specify either an initial_value, or a
missing callback, but not both).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia