[Python-Dev] PEP 567 v2

[Guido van Rossum]

On Wed, Jan 3, 2018 at 12:32 PM, Glenn Linderman <v+python at g.nevcal.com>
wrote:

> On 1/3/2018 11:16 AM, Guido van Rossum wrote:
>
> Maybe I should clarify again what run() does. Here's how I think of it in
> pseudo code:
>
> def run(self, func, *args, **kwds):
>     old = _get_current_context()
>     new = old.copy()
>     _set_current_context(new)
>     try:
>         return func(*args, **kwds)
>     finally:
>         _set_current_context(old)
>
> </lurking>
>
> I find it interesting that self isn't used in the above pseudo-code. I
> thought that Context.run() would run the function in the "context" of self,
> not in the context of a copy of "current context".
>
> </lurking>
>

Heh, you're right, I forgot about that. It should be more like this:

def run(self, func, *args, **kwds):
    old = _get_current_context()
    _set_current_context(self)  # <--- changed line
    try:
        return func(*args, **kwds)
    finally:
        _set_current_context(old)

This version, like the PEP, assumes that the Context object is truly
immutable (not just in name) and that you should call it like this:

contextvars.copy_context().run(func, <args>)

The PEP definitely needs to be clearer about this -- right now one has to
skip back and forth between the specification and the implementation (and
sometimes the introduction) to figure out how things really work. Help is
wanted!

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