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

Scott Dial scott+python-dev at scottdial.com
Fri Mar 27 08:28:55 CET 2009

P.J. Eby wrote:
> One remaining quirk or missing piece: ISTM there needs to be a way to
> extract the return value without using a yield-from statement.  I mean,
> you could write a utility function like:
>    def unyield(geniter):
>        try:
>            while 1: geniter.next()
>        except GeneratorReturn as v:
>            return v.value

My first thought was to ask why it was not equivalent to say:

    x = yield g
    x = yield from g

This would seem like a more obvious lack of parallelism to pick on wrt.
return values.

This unyield() operation seems contrived. Never before have you been
able to write a generator that returns a value, why would these suddenly
become common practice? The only place a return value seems useful is
when refactoring a generator and you need to mend having loss of a
shared scope. What other use is there for a return value?

It would seem unfortunate for it to be considered a runtime error since
this would prevent sharing a generator amongst "yield from" and
non-"yield from" use cases. Although, it would be trivial to do:

    class K:
        def _f():
            yield 1
            return 2 # used internally
        def f()
            # squelch the runtime error
            yield from self._f()

As Greg has said a number of times, we allow functions to return values
with them silently being ignored all the time.

Scott Dial
scott at scottdial.com
scodial at cs.indiana.edu

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