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

[Paul Moore]

On 23 November 2017 at 12:28, Ivan Levkivskyi <levkivskyi at gmail.com> wrote:
> On 23 November 2017 at 13:11, Paul Moore <p.f.moore at gmail.com> wrote:
>>
>> On 23 November 2017 at 12:01, Ivan Levkivskyi <levkivskyi at gmail.com>
>> wrote:
>>
>> > "I don't use it, therefore it is not needed"  is a great argument,
>> > thanks.
>> > Lets just forget about two SO questions and dozens people who up-voted
>> > it.
>> > Do you use async comprehensions? If not, then we don't need them either.
>>
>> For those of us trying to keep up with the discussion who don't have
>> time to chase the various references, and in the interest of keeping
>> the discussion in one place, can you summarise the real-world use
>> cases from the SO questions here? (I assume they are real world cases,
>> and not just theoretical questions)
>
>
> OK, here are the links:
>
> https://stackoverflow.com/questions/45190729/differences-between-generator-comprehension-expressions
> https://stackoverflow.com/questions/29334054/why-am-i-getting-different-results-when-using-a-list-comprehension-with-coroutin
> https://bugs.python.org/issue10544
> https://bugs.python.org/issue3267
>
> In all four cases pattern is the same, people were trying to refactor
> something like this:
>
> def f():
>     res = []
>     for x in y:
>         r = yield x
>         res.append(r)
>     return res
>
> into something like this:
>
> def f():
>     return [(yield x) for x in y]
>
> My understanding is that none of the case is _pressing_, since they all
> start with a for-loop, but
> following this logic comprehensions themselves are not needed. Nevertheless
> people use them because they like it.
> The problem in all four cases is that they got hard to debug problem, since
> calling `f()` returns a generator,
> just not the one they would expect.

OK, thanks. I can see why someone would want to do this. However, it
seems to me that the problem (a hard to debug error) could be solved
by disallowing yield in comprehensions and generator expressions
(giving an *easy* to debug error). I don't think the above is a
compelling argument that we have to support the one-line form. If
there was a non-trivial body of actual user code that uses the loop
form, which would be substantially improved by being able to use
comprehensions, that would be different. To put it another way, the
example you gave is still artificial. The second link is a real use
case, but as you say seems to be more a question about "why did this
not work as I expected" which could be solved with a SyntaxError
saying "yield expression not allowed in comprehensions".

Paul