Yield

Fredrik Lundh fredrik at pythonware.com
Wed Nov 15 18:25:54 CET 2006


Mateuszk87 wrote:

> may someone explain "yield" function, please. how does it actually work
> and when do you use it?

it returns a value from a function without actually terminating the 
function; when the function is resumed, it'll continue to execute after 
the yield.

a function that contains a yield statement is called a "generator", and 
is most often used in a for-in loop, or in other contexts that expect a 
sequence.  the loop is automatically terminated when the function 
returns in a usual way:

 >>> def gen():
...     yield 1
...     yield 2
...     yield 3
...
 >>> for item in gen():
...     print item
...
1
2
3
 >>> sum(gen())
6
 >>> [str(i) for i in gen()]
['1', '2', '3']

you can also use the generator "by hand"; when you call a generator 
function, it returns a special "generator object", and then immediately 
suspends itself.  to run the generator, call its "next" method:

 >>> g = gen()
 >>> g
<generator object at 0x00AE64E0>
 >>> g.next()
1
 >>> g.next()
2
 >>> g.next()
3

when the generator is exhausted, it raises a StopIterator exception:

 >>> g.next()
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
StopIteration

reference information:

     http://effbot.org/pyref/yield.htm

hope this helps!

</F>




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