[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Nathaniel Smith]

On Fri, Nov 24, 2017 at 4:22 PM, Guido van Rossum <guido at python.org> wrote:
> The more I hear about this topic, the more I think that `await`, `yield` and
> `yield from` should all be banned from occurring in all comprehensions and
> generator expressions. That's not much different from disallowing `return`
> or `break`.

I would say that banning `yield` and `yield from` is like banning
`return` and `break`, but banning `await` is like banning function
calls. There's no reason for most users to even know that `await` is
related to generators, so a rule disallowing it inside comprehensions
is just confusing. AFAICT 99% of the confusion around async/await is
because people think of them as being related to generators, when from
the user point of view it's not true at all and `await` is just a
funny function-call syntax.

Also, at the language level, there's a key difference between these
cases. A comprehension has implicit `yield`s in it, and then mixing in
explicit `yield`s as well obviously leads to confusion. But when you
use an `await` in a comprehension, that turns it into an async
generator expression (thanks to PEP 530), and in an async generator,
`yield` and `await` use two separate, unrelated channels. So there's
no confusion or problem with having `await` inside a comprehension.

-n

-- 
Nathaniel J. Smith -- https://vorpus.org