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Doubt on generators behavior

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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.

>>> 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

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Doubt on generators behavior Krishnan Shankar <i.am.songoku@gmail.com> - 2013-10-13 13:49 +0530
  Re: Doubt on generators behavior Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-10-13 11:39 +0300
  Re: Doubt on generators behavior Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-13 09:14 +0000

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