[Python-ideas] Revised**10 PEP on Yield-From

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Apr 16 04:35:02 CEST 2009

Erik Groeneveld wrote:

> By "closing the delegating generator" you mean "from the outside, call
> close() on it"?

Yes, that's right.

> I assume that 'return 1 2 3' will have one return value being a tuple
> (1,2,3)

Um, you can't write 'return 1 2 3'. You can write
'return 1, 2, 3' which returns a tuple.

>> For convenience, the ``StopIteration`` exception will be given a
>> ``value`` attribute that holds its first argument, or None if there
>> are no arguments.
> I am using StopIteration's 'args' atrribute?

The 'value' attribute is purely a convenience -- you
can get it from args[0] just as well.

> For clearity, you could probably add the name of the refactoring, it
> is called 'extract method' isn't it?

Quite likely it's called that in some book or other,
I wouldn't really know. I don't think of refactorings
as having names, I just do them.

> Yes, I believe you make sure that:
> try:
>     x = yield from y()
> except SomeError:
>    return 'HELP'
> actually does catch the SomeError exception when raised in y(), or one
> it its descendants?

That's the idea.

>> Originally it was proposed to simply extend StopIteration to accept
>> a value...

That whole paragraph shouldn't be there, it's left over
from an earlier version. Guido originally wanted to use
a different exception for returning with a value, but
he changed his mind.

> Will the the delegating generator remain on the call-stack or not?

It certainly shows up in tracebacks, but I'm not
actually sure whether it will be seen by
sys._getframe() while a subiterator is running
in my current implementation. I'll try an
experiment to find out.

Even if it does, it's probably not something you
should rely on, since a different implementation
that optimizes more aggressively could behave

In the case of thread-local storage for generator
based threads, I don't think I would try to attach
them to a generator frame, because a thread isn't
just a single generator, it's a collection of

Rather I'd use the scheduler to manage them. The
scheduler knows when it's switching from one
thread to another and can switch in the appropriate
set of variables.


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