[Python-Dev] PEP 334 - Simple Coroutines via SuspendIteration (original) (raw)

Armin Rigo arigo at tunes.org
Thu Sep 9 12:14:44 CEST 2004

Hi,

I agree with Samuele that the proposal is far too vague currently. You should try to describe what precisely should occur in each situation.

A major problem I see with the proposal is that you can describe what should occur in some situations by presenting source code snippets; such descriptions correspond easily to possible semantics at the bytecode level. But bytecode is not a natural granularity for coroutine issues. Frames (either of generators or functions) execute operations that may invoke new frames, and all frames in the chain except possibly the most recent one need to be suspended during the execution of their current bytecode. For example, a generator f() may currently be calling a generator g() with a FOR_ITER bytecode ('for' statement), a CALL_FUNCTION (calling next()), or actually anything else like a BINARY_ADD which calls a nb_add implemented in C which indirectly calls back to Python code.

For this reason it is not reasonably possible to implement restartable exceptions in general: when an exception is caught, not all the C state is saved (i.e. you don't know where, within the execution of a bytecode, you should restart). Your PEP is very similar to restartable exceptions: their possible semantics are difficult to specify in general. You may try to do that to understand what I mean.

This doesn't mean that it is impossible to figure out a more limited concept, like you are trying to do. However keeping the "restartable exception" idea in mind should help focusing on the difficult problems and where restrictions are needed.

I think that Stackless contains all the solutions in this area, and I'm not talking about the C stack hacking. Stackless is sometimes able to switch coroutines without hacking at the C stack. I think that if any coroutine support is ever going to be added to CPython it will be done in a similar way.
(Generators were also inspired by Stackless, BTW.) (Also note that although the generator syntax is nice and helpful, it would have been possible to write generators without any custom 'yield' syntax if we had restartable exceptions; this makes the latter idea more general and independent from generators.)

A bientôt,

Armin.

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