On 21Sep2008 18:36, Fredrik Lundh <[EMAIL PROTECTED]> wrote: > Roy Smith wrote: >> There are plausible examples of collections which grow while you're >> iterating over them. I'm thinking specifically of a queue in a >> multi-threaded application. One thread pushes work onto the back of >> the queue while another pops from the front. The queue could certainly >> go empty at times. But, maybe a Python iterator is just the wrong way >> to model such behavior. > > you probably want the consumer thread to block when it catches up with > the producer, rather than exit. > (that's the default behaviour of Python's Queue object, btw)
It sounds like he wants non-blocking behaviour in his consumer. A common example is "try to gather a lot of stuff into a single packet, but send a smaller packet promptly if there isn't much stuff". You could make the next() method return a sentinal value like None when the queue is empty. That would mean your consumer must recognise the special value and also precludes delivering that value through the queue. I'm not convinced my suggestion here is any better than just doubling up every call to next() with an empty() check immediately beforehand. You could write a trivial wrapping generator to take the original blocking queue and return a sentinel value on empty, too. My suggestion is also an excellent way of getting programs that fail-busy (i.e. they spin out) if you make a logic error in your consumer. Ouch. Cheers, -- Cameron Simpson <[EMAIL PROTECTED]> DoD#743 http://www.cskk.ezoshosting.com/cs/ Kill, v.t. To create a vacancy without nominating a successor. Ambrose Bierce (1842-1914), U.S. author. The Devil's Dictionary (1881-1906). -- http://mail.python.org/mailman/listinfo/python-list
