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

[Antoine Pitrou]

On Wed, 22 Nov 2017 15:03:09 +0200
Serhiy Storchaka <storchaka at gmail.com> wrote:
>  From 
> https://stackoverflow.com/questions/45190729/differences-between-generator-comprehension-expressions.
> 
>      g = [(yield i) for i in range(3)]
> 
> Syntactically this looks like a list comprehension, and g should be a 
> list, right? But actually it is a generator. This code is equivalent to 
> the following code:
> 
>      def _make_list(it):
>          result = []
>          for i in it:
>              result.append(yield i)
>          return result
>      g = _make_list(iter(range(3)))
> 
> Due to "yield" in the expression _make_list() is not a function 
> returning a list, but a generator function returning a generator.
> 
> This change in semantic looks unintentional to me. It looks like leaking 
> an implementation detail.

Perhaps we can deprecate the use of "yield" in comprehensions and make
it a syntax error in a couple versions?

I don't see a reason for writing such code rather than the more
explicit variants.  It looks really obscure, regardless of the actual
semantics.

Regards

Antoine.