[Python-Dev] PEP 567 v2

[Guido van Rossum]

On Fri, Jan 5, 2018 at 2:43 PM, Chris Jerdonek <chris.jerdonek at gmail.com>
wrote:

> On Fri, Jan 5, 2018 at 8:29 AM, Guido van Rossum <guido at python.org> wrote:
> > On Fri, Jan 5, 2018 at 2:05 AM, Victor Stinner <victor.stinner at gmail.com
> >
> > wrote:
> >>
> >> Currently, Context.get(var) returns None when "var in context" is false.
> >> That's surprising and different than var.get(), especially when var has
> a
> >> default value.
> >
> > I don't see the problem. Context.get() is inherited from Mapping.get();
> if
> > you want it to raise use Context.__getitem__() (i.e. ctx[var]). Lots of
> > classes define get() methods with various behaviors. Context.get() and
> > ContextVar.get() are just different -- ContextVar is not a Mapping.
>
> One thing that I think could be contributing to confusion around the
> proposed API is that there is a circular relationship between Context
> and ContextVar, e.g. ContextVar.get() does a lookup in the current
> Context with "self" (the ContextVar object) as a key....
>
> Also, it's the "keys" (the ContextVar objects) that have the get()
> method that should be used rather than the container object (the
> Context). This gives the confusing *feeling* of a mapping of mappings.
> This is different from how the containers people are most familiar
> with work -- like dict.
>

Only if you think of ContextVar as a mapping, which is not at all how it
works. (If anything, its get() method is more like that on weak references.)


> Is there a reason ContextVar needs to be exposed publicly at all?  For
> example, the API could use string keys like contextvars.get(name) or
> Context.get(name) (class method). There could be separate functions to
> initialize keys with desired default values, etc (internally creating
> ContextVars as needed).
>
> If the issue is key collisions, it seems like this could be handled by
> namespacing or using objects (namespaced by modules) instead of
> strings.
>
> Maybe this approach was ruled out early on in discussions, but I don't
> see it mentioned in the PEP.
>

Yes this was litigated repeatedly. Maybe the PEP 550 discussion or Rejected
Ideas section have more.

-- 
--Guido van Rossum (python.org/~guido)
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