On 26/02/2013 12:07 PM, Vytas D. wrote:
Hi,You are using "yield" incorrectly. "yield" works like return, but it can return more than once from the same function. Functions that "yield" produce a so called "generator" object. This generator object gives you values every time you call it. The generator works very interesting way. It starts like normal function and goes until it finds "yield" and returns the value. The state of generator is saved - it is like it is put to sleep until you call it again. So the next time you call generator() it runs from the point it returned last time and will return you another value. Simple sample of making and using generator (prints forever, so just kill with CTRL+C). def counter(start_at=0): """Returns integer each time called""" count = start_at while True: yield count count += 1 def main(): generator = counter() while True: print(next(generator)) if __name__ == '__main__': main() Hope helps. Vytas D. On Tue, Feb 26, 2013 at 4:34 PM, Colin J. Williams <c...@ncf.ca <mailto:c...@ncf.ca>> wrote: On 24/02/2013 7:36 PM, Ziliang Chen wrote: Hi folks, When I am trying to understand "yield" expression in Python2.6, I did the following coding. I have difficulty understanding why "val" will be "None" ? What's happening under the hood? It seems to me very time the counter resumes to execute, it will assign "count" to "val", so "val" should NOT be "None" all the time. Thanks ! code snippet: ---- def counter(start_at=0): count = start_at while True: val = (yield count) if val is not None: count = val else: print 'val is None' count += 1 Perhaps it's becaoue (teild count) is a statement. Statements do not return a value. Colin W. -- http://mail.python.org/__mailman/listinfo/python-list <http://mail.python.org/mailman/listinfo/python-list>
Yes, it's very helpful. Thanks also to the other two responders. This brings us back to the OP question. Why not " val = (yield count)"? Colin W. -- http://mail.python.org/mailman/listinfo/python-list
