[Paul Rubin] > ... > When I try to do it in a separate thread: > > import time, itertools > def remote_iterate(iterator, cachesize=5): > # run iterator in a separate thread and yield its values > q = Queue.Queue(cachesize) > def f(): > print 'thread started' > for x in iterator: > q.put(x) > threading.Thread(target=f).start() > while True: > yield q.get() > > g = remote_iterate(itertools.count)
You didn't run this code, right? itertools.count() was intended. In any case, as when calling any generator, nothing in the body of remote_iterate() is executed until the generator-iterator's next() method is invoked. Nothing here does that. So, in particular, the following is the _only_ line that can execute next: > print 'zzz...' And then this line: > time.sleep(3) And then this: > print 'hi' And then this: > for i in range(5): And then the first time you execute this line is the first time any code in the body of remote_iterate() runs: > print g.next() > > I'd expect to see 'thread started' immediately, then 'zzz...', then a 3 > second pause, then 'hi', then the numbers 0..4. Instead, the thread > doesn't start until the 3 second pause has ended. That's all as it must be. > When I move the yield statement out of remote_iterate's body and > instead have return a generator made in a new internal function, it > does what I expect: > > import time, itertools > def remote_iterate(iterator, cachesize=5): Note that remote_iterate() is no longer a generator, so its body is executed as soon as it's called. > # run iterator in a separate thread and yield its values > q = Queue.Queue(cachesize) > def f(): > print 'thread started' > for x in iterator: > q.put(x) > threading.Thread(target=f).start() And so the thread starts when remote_iterate() is called. > def g(): > while True: > yield q.get() > return g() > > Any idea what's up? Is there some race condition, where the yield > statement freezes the generator before the new thread has started? No. > Or am I just overlooking something obvious? No, but it's not notably subtle either ;-) -- http://mail.python.org/mailman/listinfo/python-list
