Doubt on generators behavior

[Terry Reedy]

On 10/13/2013 4:19 AM, Krishnan Shankar wrote:
> Hi Friends,
>
> I am new to Generators and was learning the same by experimenting. I
> wrote a small code which is as below.
>
>  >>> def test_gen(var):
> ...     print "The number is", var
> ...     if var % 2 == 0:
> ...         yield var
> ...     else:
> ...         print "Number is odd"
> ...
>  >>>
>
> But when i was executing i saw a behavior i could not understand. When i
> gave an even number,
> 1. The generator object was created
> 2. When i gave next the argument was yielded.
> 3. In the subsequent next the Stop Iteration was raised.
>
>  >>> res = test_gen(78)
>  >>> res.next()
> The number is 78
> 78
>  >>> res.next()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> StopIteration
>
> But When i ran with an odd number the result of "Number is odd" is
> printed. But it seems the generator runs again by itself to Stop Iteration.

When number is odd, there is nothing to yield, so it raises 
StopIteration on the first next call.

>  >>> res2 = test_gen(77)
>  >>> res2.next()
> The number is 77
> Number is odd
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> StopIteration
>  >>>
>
> How did this happen automatically? I am not able to get the execution of
> a generator. Can someone please help me in understanding?
> Regards,
> Krishnan
>
>


-- 
Terry Jan Reedy