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

[Chris Angelico]

On Fri, Nov 24, 2017 at 1:21 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> On 23 November 2017 at 13:04, Ivan Levkivskyi <levkivskyi at gmail.com> wrote:
>> 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 :-)
>
> My main concern is that comprehension is not equivalent to a for loop
> for a specific reason - the scope issue. Has anyone looked back at the
> original discussions to confirm *why* a function was used?
>
> My recollection:
>
>>>> i = 1
>>>> a = [i for i in (1,2,3)]
>>>> print(i)
> 1
>
> Serihy's approach (and your described expansion) would have print(i)
> return NameError.
>
> So - do we actually have a proposal to avoid the implied function that
> *doesn't* break this example? I'm pretty sure this was a real-life
> issue at the time we switched to the current implementation.

A while back I had a POC patch that made "with EXPR as NAME:" create a
new subscope with NAME in it, such that the variable actually
disappeared at the end of the 'with' block. Should I try to track that
down and adapt the technique to comprehensions? The subscope shadows
names exactly the way a nested function does, but it's all within the
same function.

ChrisA