[Python-ideas] PEP 572 version 2: Statement-Local Name Bindings

[Chris Angelico]

On Sat, Mar 24, 2018 at 11:06 PM, Paul Moore <p.f.moore at gmail.com> wrote:
> On 24 March 2018 at 11:55, Chris Angelico <rosuav at gmail.com> wrote:
>> On Sat, Mar 24, 2018 at 10:49 PM, Paul Moore <p.f.moore at gmail.com> wrote:
>>> On 24 March 2018 at 09:18, Chris Angelico <rosuav at gmail.com> wrote:
>>>> Except that a list comprehension is implemented using an inner
>>>> function. Very approximately:
>>>>
>>>> x = [n * m for n in range(4) for m in range(5)]
>>>>
>>>> def <listcomp>(iter):
>>>>     ret = []
>>>>     for n in iter:
>>>>         for m in range(5):
>>>>             ret.append(n * m)
>>>>     return ret
>>>> x = <listcomp>(iter(range(4))
>>>>
>>>> So the first (outermost) iterable is actually evaluated in the
>>>> caller's scope, but everything else is inside a subscope. Thus an
>>>> assignment inside that first iterable WILL leak into the surrounding
>>>> scope; but anywhere else, it won't.
>>>
>>> Wow, that's subtle (in a bad way!). I'd much rather that assignments
>>> don't leak at all - that seems to me to be the only correct design,
>>> although I understand that implementation practicalities mean it's
>>> hard to do.
>>>
>>> There's a lot of context snipped here. Is this about the variant that
>>> just does assignment without the new scope? If it is, then is there a
>>> similar issue with the actual proposal, or is that immune to this
>>> problem (I suspect that it's not immune, although the details may
>>> differ).
>>
>> The code equivalence I gave above applies to the existing Python
>> semantics. I'm not sure why the outermost iterable is evaluated in its
>> enclosing context, but there must have been a good reason.
>
> Understood. But the current Python semantics doesn't have any way of
> binding a name as part of that outermost iterable. It's the
> interaction of the new feature (name binding in expressions) with the
> existing implementation (the first iterable is outside the constructed
> scope for the comprehension) that needs to be clarified and if
> necessary modified to avoid nasty consequences.

And that's one reason that these assignment expressions make more
sense as statement-locals. It'd effectively still be local to the list
comp, even though it's in the outer scope.

ChrisA