[Jeremy]
Are there non-trivial examples? The PEP suggests that they exist, but doesn't provide any.
Well, this example *should* be both easy and natural -- but it turns out to be a disaster on two distinct counts if late binding is used: http://mail.python.org/pipermail/python-dev/2003-October/039323.html I'll repeat one bit from http://mail.python.org/pipermail/python-dev/2003-October/039328.html too: 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. [If] 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>).
Note that Armin Rigo suggested a small change here a few weeks ago that makes the list comp and the gen expr behave the same way. The range(i) part of the gen expr is evaluated at the point of definition. The gen expr, thus, translates to:
def f(it): for x in it: yield x * 2
iterators.append(f(range(i))
As a result of this change, the only new scope is for the body of the target expression. I don't know how that effects the earlier examples.
The example in the first link above is: pipe = source for p in predicates: # add a filter over the current pipe, and call that the new pipe pipe = e for e in pipe if p(e) "p" and "pipe" both vary, and disaster ensues if the bindings (for both) in effect at the time of each binding (to "pipe") aren't used when the last generator in the chain (the final binding of "pipe") is provoked into delivering results. If Armin's suggestion transforms that to pipe = source for p in predicates: def g(it): for e in it: if p(e): yield e pipe = g(pipe) then the binding of "p" is left bound to predicates[-1] in all the intermediate generators.
BTW is there good terminology for describing the parts of a list comp or gen expr? How about the iterator, the conditions, and the expression?
Probably more descriptive than body, mind and soul <wink>. Fine by me!