On Thursday 23 October 2003 04:12 am, Phillip J. Eby wrote:
At 02:49 PM 10/23/03 +1300, Greg Ewing wrote:
This would allow the current delayed-evaluation semantics to be kept as the default, while eliminating any need for using the default-argument hack when you don't want delayed evaluation.
Does anybody actually have a use case for delayed evaluation? Why would you ever *want* it to be that way? (Apart from the BDFL's desire to have the behavior resemble function behavior.)
I have looked far and wide over my code, present and desired, and can only find one example that seems perhaps tangentially relevant -- and I don't think it's a _good_ example. Anyway, here it comes: def smooth(N, sequence): def fifo(N): window = [] while 1: if len(window) < N: yield None else: yield window window.pop(0) window.append(item) latest = iter(fifo(N)).next for item in sequence: window = latest() if window is None: continue yield sum(window) / N as I said, I don't like it one bit; the non-transparent "argument passing" of item from the loop "down into" the generator is truly yecchy. There are MUCH better ways to do this, such as def fifo(N, sequence): it = iter(sequence) window = list(itertools.islice(it, N)) while 1: yield window window.pop(0) window.append(it.next()) def smooth(N, sequence): for window in fifo(N, sequence): yield sum(window) / N It's not clear that this would generalize to generator expressions, anyway. But I could imagine it might, e.g. IF we had "closure semantics" rather than "snapshot-binding" somebody COULD be tempted to such murky cases of "surreptitious argumentpassing down into genexprs"... and we're better off without any such possibility, IMHO.
And, if there's no use case for delayed evaluation, why make people jump through hoops to get the immediate binding?
I understand Guido's position that simplicity and regularity of the rules count (a LOT). But in this case I think Tim's insistence on practicality should count for more: the "bind everything at start time" semantics are NOT a weird special case, and the "lookup everything each time around the loop" ones don't seem to yield any non-weird use... Alex