[Python-Dev] Proposal for 2.5: Returning values from PEP 342 enhanced generators (original) (raw)

Christopher Armstrong radeex at gmail.com
Tue Oct 4 04:03:41 CEST 2005

On 10/4/05, Piet Delport <pjd at satori.za.net> wrote:

One system that could benefit from this change is Christopher Armstrong's defgen.py[1] for Twisted, which he recently reincarnated (as newdefgen.py) to use enhanced generators. The resulting code is much cleaner than before, and closer to the conventional synchronous style of writing.

[1] the saga of which is summarized here: http://radix.twistedmatrix.com/archives/000114.html However, because enhanced generators have no way to differentiate their intermediate results from their "real" result, the current solution is a somewhat confusing compromise: the last value yielded by the generator implicitly becomes the result returned by the call. Thus, to return something, in general, requires the idiom "yield Foo; return". If valued returns are allowed, this would become "return Foo" (and the code implementing defgen itself would probably end up simpler, as well).

Hey, that would be nice. I've found people confused by the way defgen handles return values before, getting seemingly meaningless values out of their defgens (if the defgen didn't specifically yield some meaningful value at the end).

At first I thought "return foo" in a generator ought to be equivalent to "yield foo; return", but at least for defgen, it turns out raising StopIteration(foo) would be better, as I would have a very explicit way to specify and find the return value of the generator.

-- Twisted | Christopher Armstrong: International Man of Twistery Radix | -- http://radix.twistedmatrix.com | Release Manager, Twisted Project \\V/// | -- http://twistedmatrix.com |o O| | w----v----w-+

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