On 22 November 2017 at 18:15, Paul Moore <[email protected]> wrote:
> On 22 November 2017 at 16:47, Ivan Levkivskyi <[email protected]> > wrote: > > On 22 November 2017 at 17:43, Paul Moore <[email protected]> wrote: > >> > >> On 22 November 2017 at 16:30, Ivan Levkivskyi <[email protected]> > >> wrote: > >> > On 22 November 2017 at 17:24, Antoine Pitrou <[email protected]> > >> > wrote: > >> >> Given a comprehension (e.g. list comprehension) is expected to work > >> >> nominally as `constructor(generator expression)` > >> > > >> > As Yury just explained, these two are not equivalent if there is an > >> > `await` > >> > in the comprehension/generator expression. > >> > >> As Antoine said, people *expect* them to work the same. > > > > > > The difference is that a generator expression can be used independently, > one > > can assign it to a variable etc. not necessary to wrap it into a list() > > Anyway, can you propose an equivalent "defining" code for both? > Otherwise it > > is not clear what you are defending. > > [...snip...] > > 1. List comprehensions expand into nested for/if statements in the > "obvious" way - with an empty list created to start and append used to > add items to it. > 1a. Variables introduced in the comprehension don't "leak" (see below). > 2. Generator expressions expand into generators with the same "nested > loop" behaviour, and a yield of the generated value. > 3. List comprehensions are the same as list(the equivalent generator > expression). > Great, I agree with all three rules. But there is a problem, it is hard to make these three rules consistent in some corner cases even _without async_. For example, with the original problematic example, it is not clear to me how to apply the rule 2 so that it is consistent with 3: def fun_comp(): return [(yield i) for i in range(3)] def fun_gen(): return list((yield i) for i in range(3)) I think the solution may be to formulate the rules in terms of the iterator protocol (__iter__ and __next__). I will try to think more about this. -- Ivan
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