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C callbacks ? Re: Is Stackless Python DEAD?

Gordon McMillan gmcm at hypernet.com
Tue Nov 6 14:02:11 EST 2001 
Frederic Giacometti wrote:

> 
> "Gordon McMillan" <gmcm at hypernet.com> wrote in message

[snip]

>> But making Python *truly* stackless means getting rid of all recursions,
>> and that is an enormous task. If you don't do that, you've got a 
>> language feature that doesn't work in some apparently random set of
>> circumstances. 
> 
> But how do you process callbacks to Python from C code (extensions or
> embeded python)?
> 
> One has to return to C after executing the Python code, and than C has to
> return to Python after executing the remainer of its code...

All depends on what you mean by "return" <wink>. Try thinking of "callbacks"
and the "return" from callbacks as events.

In Stackless, the Python stack and the C stack are completely separate.
In the case where C code needs to call Python and then do something with
the result (that is, where tail recursion doesn't apply), Christian used
the trick of manufacturing a Python-style frame object that represents the C
code. As far as (Stackless) Python is concerned, it gets dispatched like
any other frame (Stackless' Python "stack" is really a tree).
 
> I see maintaining hybrid behavior (iterative / recursive calls to the
> interpreter, and 'recursion tolerance') as a requirement...

True. And Stackless (as a 3rd party product) can get away with saying 
"Oops, I can't do that". The bar is higher for a core language feature,
though. Which is a shame, because this turns out to be more of a 
theoretical problem than a real one.

- Gordon
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