Jack Jansen wrote:

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Let me make sure I understand: If I invoke a continuation when there are extra C stack frames between the mainloop invocation that captured the continuation and the call of the continuation, the interpreter raises an exception?

Not always. Frames which are not currently bound by an interpreter acting on them can always be jump targets. Only those frames which are currently in the middle of an opcode are forbidden.

And how about the reverse? If I'm inside a Python callback from C code, will the Python code be able to use continuations? This is important, because there are a lot of GUI applications where almost all code is executed within a C callback. I'm pretty sure (and otherwise I'll be corrected within milliseconds:-) that this is the case for MacPython IDE and PythonWin (don't know about Idle).

Without extra effort, this will be problematic. If C calls back into Python, not by the trampoline scheme that stackless uses, but by causing an interpreter recursion, then this interpreter will be limited. It can jump to any other frame that is not held by an interpreter on the C stack, but the calling frame of the C extension for instance is locked. Touching it causes an exception. This need not necessarily be a problem. Assume you have one or a couple of frames sitting around, caught as a continuation. Your Python callback from C jumps to that continuation and does something. Afterwards, it returns to the C callback. Performing some cycles of an idle task may be a use of such a thing. But as soon as you want to leave the complete calling chain, be able to modify it, return to a level above your callback and such, you need to implement your callback in a different way. The scheme is rather simple and can be seen in the stackless map implementation: You need to be able to store your complete state information in a frame, and you need to provide an execute function for your frame. Then you return the magic Py_UnwindToken, and your prepared frame will be scheduled like any pure Python function frame. Summary: By default, C extensions are restricted to stackful behaviror. By giving them a stackless interface, you can enable it completely for all continuation stuff. cheers - chris -- Christian Tismer :^) <mailto:tismer@appliedbiometrics.com> Applied Biometrics GmbH : Have a break! Take a ride on Python's Kaunstr. 26 : *Starship* http://starship.python.net 14163 Berlin : PGP key -> http://wwwkeys.pgp.net PGP Fingerprint E182 71C7 1A9D 66E9 9D15 D3CC D4D7 93E2 1FAE F6DF where do you want to jump today? http://www.stackless.com