(This is drawing to a conclusion. Summary: Tim has convinced me.)
There are other places in Python where some rule is applied to "all free variables of a given piece of code" (the distinction between locals and non-locals in functions is made this way). But there are no other places where implicit local *copies* of all those free variables are taken.
I didn't suggest to copy anything, just to capture the bindings in use at the time a generator expression is evaluated.
Sorry, I meant a pointer copy, not an object copy. That's a binding capture.
This is easy to explain, and trivial to explain for people familiar with the default-argument trick.
Phillip Eby already recommended not bothering with that; the default-argument rule is actually confusing for newbies (they think the defaults are evaluated at call time) so it's best not to bring this into the picture.
Whenever I've written a list-of-generators, or in the recent example a generator pipeline, I have found it semantically necessary, without exception so far, to capture the bindings of the variables whose bindings wouldn't otherwise be invariant across the life of the generator. It it turns out that this is always, or nearly almost always, the case, across future examples too, then it would just be goofy not to implement generator expressions that way ("well, yes, the implementation does do a wrong thing in every example we had, but what you're not seeing is that the explanation would have been a line longer had the implementation done a useful thing instead" <wink>).
I'd need to find a unifying principle to warrant doing that beyond utility.
No you don't -- you just think you do <wink>.
OK, I got it now. I hope we can find another real-life example; but there were some other early toy examples that also looked quite convincing. I'll take a pass at updating the PEP. --Guido van Rossum (home page: http://www.python.org/~guido/)