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

Guido van Rossum guido at python.org
Fri Jan 5 18:00:11 EST 2018

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.

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