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

[Ivan Levkivskyi]

On 23 November 2017 at 13:45, Paul Moore <p.f.moore at gmail.com> wrote:

> 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)
> [...]
> >
> > 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".
>

The level of "artificialness" is quite subjective, this is rather matter of
taste (see the tornado example).
Let us forget for a moment about other problems and focus on this one: list
comprehension is currently not equivalent to a for-loop.
There are two options:
- Fix this, i.e. make comprehension equivalent to a for-loop even in edge
cases (Serhiy seems ready to do this)
- Prohibit all cases when they are not equivalent

I still prefer option one. But I see your point, option two is also an
acceptable fix.
Note that there were not so many situations when some code became
SyntaxError later.
I don't see why this particular case qualifies for such a radical measure
as an exception to syntactic rules,
instead of just fixing it (sorry Nick :-)

--
Ivan
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