[Python-Dev] PEP 380 (yield from a subgenerator) comments

[Guido van Rossum]

On Fri, Mar 27, 2009 at 12:53 AM, Greg Ewing
<greg.ewing at canterbury.ac.nz> wrote:
> Guido van Rossum wrote:
>
>> That +0 could turn into a +1 if there was a way to flag this as an
>> error (at runtime), at least if the return is actually executed:
>>
>> def g():
>>    yield 42
>>    return 43
>>
>> for x in g():
>>    print x    # probably expected to print 42 and then 43
>>
>> Perhaps the exception used in this case could be a different exception
>> than StopIteration?
>
> Would checking that the value is None be sufficient? Or do you
> want to distinguish between 'return' and 'return None'? That
> would feel rather strange to me.

Well currently there *is* a difference, because "return None" is a
(phase 2) SyntaxError in a generator, while "return" isn't. But I
don't really care if you cannot distinguish these two cases.

> I'm inclined to regard this as an unnecessary complication.
> People are already trained to think of 'return' and 'yield'
> as quite different things. I don't know why they would suddenly
> start using 'return' when they mean 'yield'.

Because coroutines are mind-bendingly hard, and it's easy to confuse
yield (pass a value back while keeping the stack frame) and return
(pass a value back dropping the stack frame). The syntactic
prohibition on "return <expr>" in generators currently helps people to
be consistent. I'd like to keep this small crutch around so that
people who are writing vanilla (== meant for iteration) generators are
kept from accidentally introducing hard-to-debug problems.

Perhaps the crux is that *if* you accidentally use "return <value>" in
a vanilla generator expecting the value to show up somewhere, you are
probably enough of a newbie that debugging this will be quite hard.
I'd like not to have such a newbie trap lying around.

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