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:

@​contextvars.caller_context
def genfunc():
    assert cvar.value is the_value
    yield
    assert cvar.value is the_value

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

next(gen)

with cvar.assign(1234567890):
    try:
        next(gen)
    except StopIteration:
        pass

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. ​​

 
but why do you say it's needed? What are these reasons that were missed? Do you have a use case?


​Yes, there's a type of use case. When you think of a generator function as a function that returns an iterable of values and you don't care about whether the values are computed lazily or not. In that case, you don't want next() or .send() to affect the context inside the generator.

​In terms of code, we might want this:

def values():
    # compute some values using cvar.value and 
    return a_list_of_values

with cvar.assign(something):
    data = values()

datalist = list(data)


...to be equivalent to:

def values():
    # compute some values using cvar.value and
    # yield them one by one

​with cvar.assign(something):
    data = values()

datalist = list(data)


So we don't want the "lazy evaluation" of generators to affect the values "in" the iterable. But I think we had our minds too deep in event loops and chains of coroutines and async generators to realize this.

Initially, this seems to do the wrong thing in many other cases, but in fact, with the right extension to this behavior, we get the right thing in almost all situtations. We still do need the other generator behaviors described in PEP 555, for async and other uses, but I would probably go as far as making this new one the default. But I kept the decorator syntax for now.  

-- Koos


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