On Mon, Oct 9, 2017 at 6:24 PM, Guido van Rossum <guido@python.org> wrote:
On Sun, Oct 8, 2017 at 11:46 PM, Nick Coghlan <ncoghlan@gmail.com> wrote:
On 8 October 2017 at 08:40, Koos Zevenhoven <k7hoven@gmail.com> wrote:
On Sun, Oct 8, 2017 at 12:16 AM, Nathaniel Smith <njs@pobox.com> wrote:
On Oct 7, 2017 12:20, "Koos Zevenhoven" <k7hoven@gmail.com> wrote:

​Unfortunately, we actually need a third kind of generator semantics, something like this:

def genfunc():
    assert cvar.value is the_value
    assert cvar.value is the_value

with cvar.assign(the_value):
    gen = genfunc()


with cvar.assign(1234567890):
    except StopIteration:

Nick, Yury and I (and Nathaniel, Guido, Jim, ...?) somehow just narrowly missed the reasons for this in discussions related to PEP 550. Perhaps because we had mostly been looking at it from an async angle.

That's certainly a semantics that one can write down (and it's what the very first version of PEP 550 did),

​​I do remember Yury mentioning that the first draft of PEP 550 captured something when the generator function was called. I think I started reading the discussions after that had already been removed, so I don't know exactly what it was. But I doubt that it was *exactly* the above, because PEP 550 uses set and get operations instead of "assignment contexts" like PEP 555 (this one) does. ​​

We didn't forget it, we just don't think it's very useful.

I'm not sure I agree on the usefulness. Certainly a lot of the complexity of PEP 550 exists just to cater to Nathaniel's desire to influence what a generator sees via the context of the send()/next() call. I'm still not sure that's worth it. In 550 v1 there's no need for chained lookups.

​We do need some sort of chained lookups, though, at least in terms of semantics. But it is possible to optimize that away in PEP 555. Some kind of chained-lookup-like thing is inevitable if you want the state not to leak though yields out of the generator:

with cvar.assign(a_value):
    # don't leak `a_value` to outer context
    yield some_stuff()


+ Koos Zevenhoven + http://twitter.com/k7hoven +