[Python-Dev] Stackless Python - Pros and Cons (original) (raw)

Christian Tismer tismer@appliedbiometrics.com
Mon, 07 Aug 2000 15:47:39 +0200

[about recursion and continuations]

CT> Right. What you see here is the incompleteness of Stackless. In CT> order to get this "right", I would have to change many parts of CT> the implementation, in order to allow for continuations in every CT> (probably even unwanted) place. I could not do this.

CT> Instead, the situation of these still occouring recursions are CT> handled differently. continuationmodule guarantees, that in the CT> context of recursive interpreter calls, the given stack order of CT> execution is obeyed. Violations of this cause simply an CT> exception. 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.

If so, continuations don't sound like they would mix well with C extension modules and callbacks. I guess it also could not be used inside methods that implement operator overloading. Is there a simple set of rules that describe the situtations where they will not work?

Right. In order to have good mixing with C callbacks, extra work is necessary. The C extension module must then play the same frame dribbling game as the eval_loop does. An example can be found in stackless map. If the C extension does not do so, it restricts execution order in the way I explained. This is not always needed, and it is no new requirement for C developers to do so. Only if they want to support free continuation switching, they have to implement it.

The simple set of rules where continuations will not work at the moment is: Generally it does not work across interpreter recursions. At least restrictions appear:

Reducing all these restrictions is a major task, and there are situations where it looks impossible without an extra subinterpreter language. If you look into the implementation of operators like add, you will see that there are repeated method calls which all may cause other interpreters to show up. I tried to find a way to roll these functions out in a restartable way, but it is quite a mess. The clean way to do it would be to have microcodes, and to allow for continuations to be caught between them.

this-is-a-stackless-3000-feature - ly y'rs - chris

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