[Python-Dev] Minimal 'stackless' PEP using generators? (original) (raw)

Phillip J. Eby pje at telecommunity.com
Mon Aug 23 21:34:13 CEST 2004

At 03:18 PM 8/23/04 -0400, Clark C. Evans wrote:

On Mon, Aug 23, 2004 at 02:59:18PM -0400, Phillip J. Eby wrote: | > def generator(): | > yield 'one value' | value, = generator() Oh, cool. ;)

| > Full-blown corountines arn't necessary. A small tweak to | > to generators will do. |>| I don't think this is true. Your hypothetical example can't resume | 'top()' after it yields the "co-operate" control value, unless it either | it has a stack of generators, or the Python core somehow maintains a | stack of the executing generators. Yes. The idea is for Python to maintain a stack of generators, and when ever the top-most generator yields this yield skips the intermediate generators on the stack, and goes immediately to the calling function.

That's not the part I mean. I'm talking about after that happens, when you resume the generators, and you'll need to reconstruct the stack as it stood when the generators were interrupted.

(Your original proposal doesn't actually mention this part at all, but I assume it's necessary. After all, if all you needed was a way to stop a bunch of generators in their tracks, raising an exception would suffice.)

| So, my point was that since this can already be done in user-level code | with a stack of generators, I don't see the point to adding a facility to | Python to create hidden stacks of generators.

The problem is, if you maintain the stack of generators in userland, then each generator on the stack must do special stuff to handle 'cooperate' (in one form or another). You demonstrated an equivalent problem with your peek.event code presented earlier. While, if this is built into the core, intermediate generators can be happily oblivious to the cooperative magic.

Though the problems are "equivalent" in complexity, the difficulty of their respective solutions are not at all comparable, because your approach either requires a technique for dynamically reconstructing the stack, or some similar mechanism for generators to keep track of the state of other generators when the containing generator is interrupted.

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