[Tim]
There's a difference, though: if `y` "leaks", BFD. Who cares? ;-) If `y` remains inaccessible, there's no way around that.
[Chris]
That's Steve D'Aprano's view - why not just let them ALL leak? I don't like it though.
I didn't suggest that. I'm not suggesting changing _any_ existing behavior (quite the contrary). Since ":=" would be brand new, there is no existing behavior for it.
... Sorry, I meant "local to the comprehension's scope". We can't know the user's intention. We have to create semantics before the user's intention even exists.
Exactly. That's why I would like ":-=" to be defined from the start in a way that does least damage ;-)
But the point above remains: if they don't leak, contexts that want them to leak have no recourse. If they do leak, then the other uses would still work fine, but they'd possibly be annoyed by a leak they didn't want.
Then let's revert the Py3 change that put comprehensions into functions, and put them back to the vanilla transformation:
Again, I'm not suggesting changing any existing behavior.
stuff = [x + 1 for x in iter if x % 3]
stuff = [] for x in iter: if x % 3: stuff.append(x + 1)
Now 'x' leaks as well, and it's more consistent with how people explain comprehensions. Is that a good thing? I don't think so. Having the iteration variable NOT leak means it's a self-contained unit that simply says "that thing we're iterating over".
It's fine by me that for-target names don't leak. I didn't suggest changing that.
Part of that is because - as the existence of this thread attests to - we can't even control all the scopes gimmicks Python already has. So people are understandably terrified of adding even more ;-)
Part of it is just that people seem to be fighting for the sake of fighting. I'm weary of it, and I'm not going to debate this point with you. You want 'em to leak? No problem. Implement it that way and I'm not going to argue it.
I'm more interested in real-life use cases than in arguments. My suggestion came from staring at my real-life use cases, where binding expressions in comprehensions would clearly be more useful if the names bound leaked. Nearly (but not all) of the time,, they're quite happy with that for-target names don't leak. Those are matters of observation rather than of argument.