3

When i was googling for python yield i found something interesting and I never knew before i.e we can pass value to yield to change the next() value. I hope some of the new pythonist might now aware of this and I am not sure how it works too. So, If any body could explain how it works and how the behavior changes when i send a new index to yield using send().

Here is my snippets :

def print_me (count):
    i = 0
    while i < count:
        val = (yield i)
        if val is not None:
            i = val
        else:
            i += 1

gen = print_me(10)

for i in gen:
   if i == 5:
       gen.send(8)
   else:
       print i

Here is my output:

0
1
2
3
4
9
James Sapam
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  • What is your question? What part of the behavior do you not understand? – BrenBarn Dec 14 '13 at 04:45
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    When i send a new index, how the yield point to new index. Basically I am confusing `yield` is using some kind of linked list or ? – James Sapam Dec 14 '13 at 04:46
  • I don't quite get what you mean by "how yield point to new index". When you use `send`, the value you send becomes the value of the `yield` expression. How you use that in the generator is up to you. – BrenBarn Dec 14 '13 at 04:47
  • This "enhanced" generator syntax was added in python 2.5 and is used to build coroutines. [read more in the PEP](http://www.python.org/dev/peps/pep-0342/) – roippi Dec 14 '13 at 04:57
  • Here's some interesting related information: http://stackoverflow.com/q/12324096/748858 – mgilson Dec 14 '13 at 04:58

1 Answers1

4

Your test code is pretty instructive actually. What happens in a generator function like this is that whenever you call next on the generator, code gets executed until it hits a yield statement. The value is yielded and then execution in the generator "pauses". If you .send some data to the generator, that value is returned from yield. If you don't .send anything, yield returns None.

So in your code, when you .send(8), then it sets val = 8 inside your generator (val = yield i). since val is not None, i is set to 8. .send actually executes until the next yield statement (returning that value -- 8). Then things resume as normal (the next number to be yielded is 9).

mgilson
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    An important thing to note is that `send` advances the generator (i.e., runs it until the next `yield` and gives you the next value). This sometimes confuses people. – BrenBarn Dec 14 '13 at 04:49
  • @BrenBarn -- Thanks, That's true (I always need to look that up). I've clarified the answer a bit. – mgilson Dec 14 '13 at 04:55
  • Thank you @BrenBarn and mgilson. I think now i got it. – James Sapam Dec 14 '13 at 04:58
  • Is there any use case for this advancing `yield` ? i din't see anything. – James Sapam Dec 14 '13 at 04:58
  • @yopy -- coroutines. I've not used them myself, so I can't really give you a good use-case. Take a look at the coroutines and info in the PEP posted by roippi. – mgilson Dec 14 '13 at 05:33