[Tim]
Since this is all about scope, while I'm not 100% sure of what Guido meant, I assumed he was saying "p can only have one scope in the synthetic function: local or non-local, not both, and local is what I propose". For example, let's flesh out his example a bit more:
p = 42 [p := p for p in range(10) if p == 3] print(p) # 42? 3? 9?
If `p` is local to the listcomp, it must print 42. If `p` is not-local, it must print 9. If it's some weird mixture of both, 3 makes most sense (the only time `p := p` is executed is when the `for` target `p` is 3).
[Jacco van Dorp <j.van.dorp@deonet.nl>]
With my limited experience, I'd consider 3 to make most sense, but 9 when thinking about it in the expanded form.
If it's not 3 tho, then the following would make most sense:
SyntaxError("Cannot re-bind for target name in a list comprehension") # Or something more clear.
And the rest of that mail that convinces me even more that an error would be the correct solution here.
Good news, then: Nick & Guido recently agreed that it would be a compile-time error. Assuming it's added to the language at all, of course.
Before I got on this mailinglist, i never even knew comprehensions introduced a new scope. I'm really that new.
They didn't, at first. That changed over time. The real reason was so that `for` variables - which people typically give little thought to naming - didn't accidentally overwrite local variables that happened to share the same name. Like:
i = -42 [i+1 for i in range(3)] [1, 2, 3] i # unchanged! -42
But you can productively use list comprehensions without knowing anything about how they're implemented, and just think "ha! Python does some happy magic for me there :-)".
Two years ago I'd look up stackoverflow to check the difference between overriding and extending a method and to verify whether I made my super() calls the right way.
If something goes to weird, I think just throwing exceptions is a sensible solution that keeps the language simple, rather than making that much of a headache of something so trivially avoided.
Since Guido agreed with you in this case, that proves you're a true Pythonista - or maybe just that you're both Dutch ;-)