Generators and exceptions (was Re: Stackless Python and Python 2.x)

[Guido van Rossum]

aahz at panix.com (Aahz Maruch) writes:

> Okay, I've re-read PEP 255, and I'm still not clear on what happens with
> the following code:
> 
> def a():
>     yield 1
>     raise "foo"
>     yield 2
>     return
> 
> def b():
>     return a()
> 
> g = b()
> g.next()
> g.next()
> 
> (If I've messed up the syntax, please feel free to correct it before
> answering; I *think* my intent is clear enough.)

You don't need the "g = b()" part -- "g = a()" has the same effect.

This prints 1 as a result of the first call to g.next(), and raises a
foo exception as a result of the second g.next() call.  After that,
any call to g.next() raises StopIteration, because after raising an
exception, the generator's frame is no longer resumable.

IOW, a raise from a generator is passed right through the next()
call, and the generator terminates for good.  Ditto for a return: this
is (mostly) equivalent to raising StopIteration.

--Guido van Rossum (home page: http://www.python.org/~guido/)