Paul Rubin wrote: > As I understand it, generators are supposed to run til they hit a > yield statement: > > import time > def f(): > print 1 > time.sleep(3) > for i in range(2,5): > yield i > > for k in f(): > print k > > prints "1" immediately, sleeps for 3 seconds, then prints 2, 3, and 4 > without pausing, as expected. 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) > print 'zzz...' > time.sleep(3) > print 'hi' > for i in range(5): > 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.
My 10-second analysis is that *none* the body of a generator runs until a value is requested from it. In [3]: def f(): ...: print 'Starting f' ...: for i in range(3): ...: yield i ...: ...: In [4]: g = f() In [5]: for i in g: ...: print i ...: ...: Starting f 0 1 2 In your first example, you instantiate the generator and then iterate over it immediately; in your second, you separate the two things. I don't think threads have anything to do with it. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
