Ideas about how software should behave (was: replacing `else` with `then` in `for` and `try`)

[Chris Angelico]

On Thu, Nov 9, 2017 at 5:05 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Wed, Nov 8, 2017 at 9:31 AM, Chris Angelico <rosuav at gmail.com> wrote:
>> On Thu, Nov 9, 2017 at 3:19 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
>>> I was not referring to the possible future use of yield from for async
>>> generators; I was referring to the possibility *today* of using "yield
>>> from" as a synonym for *await*. As far as I know the only major
>>> obstacle to that is that the authors (with good reason) made it a
>>> SyntaxError. This is exactly the same sort of situation: it's a
>>> construct that would otherwise be perfectly valid, but it's made a
>>> SyntaxError specifically to prevent users from doing some the devs
>>> don't want them to.
>>
>> I don't understand why you would use "yield from" as a synonym for
>> "await". They are not equivalent. Why would you use one in place of
>> the other?
>
> There's not really a good reason to use "yield from" with "async def"
> when you could just use "await", but the point is that in principle
> you could. In a generator-based coroutine (e.g. asyncio prior to
> Python 3.5), "yield from" is used to pause the coroutine and wait on
> some future. In a native coroutine (e.g. after Python 3.5), "await" is
> used to pause the coroutine and wait on some future. The
> implementation AIUI is essentially the same; the __await__ method is
> even required to return an iterator, just like __iter__.
>
> That's why I'm saying that they're basically synonyms. All that's
> really separating them is the syntax error.

Except that "yield from" is used by generators to delegate to other
generators, and "await" is used by coroutines to delegate to other
coroutines. In an asynchronous generator, "yield" produces values, and
"yield from" would delegate to another asynchronous generator. They
are NOT synonyms.

ChrisA