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

Ivan Levkivskyi levkivskyi at gmail.com
Wed Nov 22 10:10:11 EST 2017

On 22 November 2017 at 15:47, Paul Moore <p.f.moore at gmail.com> wrote:

> I generally don't understand "await" in any context, so I deferred
> judgement on that :-) Based on your comment that they are equally
> tricky, I'd suggest we prohibit them both ;-)
> Less facetiously, comprehensions are defined in the language reference
> in terms of a source translation to nested loops. That description
> isn't 100% precise, but nevertheless, if yield/async in a
> comprehension doesn't behave like that, I'd consider it a bug. So
> current behaviour (for both yield and await) is a bug, and your
> proposed semantics for yield is correct.

I think there may be a small misunderstanding here. The situation is
different for comprehensions and generator expressions,
let me summarize the current state:

- yield in comprehensions works "wrong" (a shorthand for not according to
the docs/naive expectations, i.e. not equivalent to for loop)
- await in comprehensions works "right"
- yield in generator expressions works "wrong"
- await in generator expressions works "wrong"

After some thinking, both `yield` and `await` look quite mind bending in
_generator expressions_, so maybe the right compromise strategy is:

- fix yield in comprehensions
- await in comprehensions already works
- make both `yield` and `await` a SyntaxError in generator expressions.

What do you think?

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