On Dec 14, 7:21 am, exar...@twistedmatrix.com wrote: > Note, I know *why* the implementation leads to this behavior. I'm > asking why "the devs" *accept* this.
As noted, the problem isn't with generators but with iteration protocol. The devs "allowed" this because it was a necessary evil for correct functionality. As the system is set up it can't discriminate between a legitimate and a spurrious StopIteration. (Which is why Steven and others called you out for suggesting that the compiler has to read your mind.) However, as far as I'm concerned there is no reasonable argument that this behavior is good. So how, hypothetically, would one go about fixing it, short of ripping out and replacing the existing machinery? The first argument is that StopIteration has no place within a generator expression. Therefore a generator expression (but not a generator function) could deliberately catch StopIteration and raise a different exception. I don't like it, though: who says that StopIteration has no place within a generator expression? Currently it's possible to do something like this to terminate a genexp early, and I won't the one saying you shouldn't do it. def stop(): raise StopIteration list(x or stop() for x in stream) (Though personally I'd bite the bullet and write it as a generator function). What else? The way I see it, when you throw StopIteration you are trying to stop a specific generator. Therefore StopIteration should (somehow) contain a reference to the generator that it's being applied to. Perhaps it can be obtained by crawling ths stack, which shouldn't be a significant penalty (it'd be only called once per iteration, and most of the time it'd be only one or two frames up). The looping logic within Python should check whether the reference matches the object it's iterating over; if not it raises a LeakyLoopException or something like that. I haven't thought this out though, I'm just kind of throwing this out there. Any issues? But to answer your question, I think "simple is better than complex" rules the day. Right now StopIteration stops an iteration, simple as that. Any fix would add complexity. Carl Banks -- http://mail.python.org/mailman/listinfo/python-list
