On Sat, Mar 24, 2018 at 11:06 PM, Paul Moore <p.f.mo...@gmail.com> wrote: > On 24 March 2018 at 11:55, Chris Angelico <ros...@gmail.com> wrote: >> On Sat, Mar 24, 2018 at 10:49 PM, Paul Moore <p.f.mo...@gmail.com> wrote: >>> On 24 March 2018 at 09:18, Chris Angelico <ros...@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 _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
