[Python-Dev] Return from generators in Python 3.2 (original) (raw)

Guido van Rossum guido at python.org
Fri Aug 27 02:25:51 CEST 2010

On Thu, Aug 26, 2010 at 5:05 PM, Yury Selivanov <yselivanov at gmail.com> wrote:

On 2010-08-26, at 8:04 PM, Greg Ewing wrote:

Even with your proposal, you'd still have to use a 'creepy abstraction' every time one of your coroutines calls another. That's why PEP 380 deals with 'more than just return'. Nope.  In almost any coroutine framework you have a scheduler or trampoline object that basically does all the work of calling, passing values and propagating exceptions.  And many other things that 'yield from' won't help you with (cooperation, deferring to process/thread pools, pausing, etc.)  Being a developer of one of such frameworks, I can tell you, that I can easily live without 'yield from', but dealing with weird return syntax is a pain.

That's not my experience. I wrote a trampoline myself (not released yet), and found that I had to write a lot more code to deal with the absence of yield-from than to deal with returns. In my framework, users write 'raise Return(value)' where Return is a subclass of StopIteration. The trampoline code that must be written to deal with StopIteration can be extended trivially to deal with this. The only reason I chose to use a subclass is so that I can diagnose when the return value is not used, but I could have chosen to ignore this or just diagnose whenever the argument to StopIteration is not None.

Especially when you use decorators like @bus.method, or @protocol.handler, that transparently wrap your callable be it generator or regular function.  And after that you have to use different return syntax for them.

Until PEP 380 is implemented, you have to use different return syntax in generators. You have some choices: raise StopIteration(value), raise SomethingElse(value), or callSomeFunction(value) -- where callSomeFunction raises the exception. I like the raise variants because they signal to tools that the flow control stops here -- e.g. in Emacs, python-mode.el automatically dedents after a 'raise' or 'return' but not after a call (of course).

-- --Guido van Rossum (python.org/~guido)

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