[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Ivan Levkivskyi levkivskyi at gmail.com
Wed Nov 22 12:36:20 EST 2017
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On 22 November 2017 at 18:15, Paul Moore <p.f.moore at gmail.com> wrote:
On 22 November 2017 at 16:47, Ivan Levkivskyi <levkivskyi at gmail.com> wrote: > On 22 November 2017 at 17:43, Paul Moore <p.f.moore at gmail.com> wrote: >> >> On 22 November 2017 at 16:30, Ivan Levkivskyi <levkivskyi at gmail.com> >> wrote: >> > On 22 November 2017 at 17:24, Antoine Pitrou <solipsis at pitrou.net> >> > 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.
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