Doubt on generators behavior

[Steven D'Aprano]

On Sun, 13 Oct 2013 13:49:53 +0530, 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.

Correct. Follow the program logic. The first time you call next, 
execution of the generator begins:

- the generator first prints "The number is 78";

- the "if" block is then checked, since 78 is an even number the
  line "yield var" is executed and 78 is yielded;

- after "yield", execution halts until you call next again;

- when you call next the second time, execution continues following 
  the if...else block, which reaches the end of the function and
  StopIteration is raised.


> 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.

Again, follow the program logic. The first time you call next, execution 
of the generator begins:

- the generator first prints "The number is 77";

- the "if" block is then checked, since 77 is an odd number the
  "else" clause is executed, and "The number is off" is printed;

- execution continues following the if...else block, which reaches
  the end of the function and StopIteration is raised.


Execution only halts at a yield statement. Try experimenting with this 
one:

def example():
    var = 3
    print "Starting with", var
    yield var  # pauses after this
    print "execution continues..."
    var -= 1
    print "var now has result", var
    yield var   # pauses after this
    var -= 1
    print "var now has result", var
    yield 1000 + var  # pauses after this
    yield "nearly done"  # pauses after this
    print "Finished now."



-- 
Steven