[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Paul Moore p.f.moore at gmail.com
Wed Nov 22 11:16:42 EST 2017

On 22 November 2017 at 16:08, Ivan Levkivskyi <levkivskyi at gmail.com> wrote:

On 22 November 2017 at 16:56, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

On Wed, Nov 22, 2017 at 10:10 AM, Ivan Levkivskyi <levkivskyi at gmail.com> wrote: > On 22 November 2017 at 15:47, Paul Moore <p.f.moore at gmail.com> wrote: [...] I'm all for prohibiting using 'yield' expression in generator expressions or comprehensions. The semantics is way to hard to understand and hence be of any value. Making 'await' a SyntaxError is absolutely not an option. Async generator expressions are a shorthand syntax for defining asynchronous generators (PEP 525), and it's already being used in the wild. OK, makes sense, so it looks like we may have the following plan: - fix yield in comprehensions

I'm still not clear what "fix" would actually mean, but you propose clarifying the docs below, so I assume it means "according to whatever the updated docs say"...

- update PEP 530 and docs re generator expressions vs comprehensions

Docs more importantly than PEP IMO. And are you implying that there's a difference between generator expressions and comprehensions? I thought both were intended to behave as if expanded to a function containing nested for loops? Nothing said in this thread so far (about semantics, as opposed to about current behaviour) implies there's a deliberate difference.

- make yield in generator expressions a SyntaxError

That contradicts the suggestion that generator expressions are equivalent to the expanded function definition we saw before.

If everyone agrees, then I propose to open a separate issue on b.p.o. to coordinate the efforts.

I agree it needs clarifying. Not sure I agree with your proposed semantics, but I guess that can wait for when we have a concrete doc change proposal. Paul

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