[Python-Dev] Stackless Python

[Bob Ippolito]

On May 31, 2004, at 5:25 PM, Martin v. Löwis wrote:

> Bob Ippolito wrote:
>
>> I'm believe that map and filter are non-recursive
>
> Can you point me to the code that demonstrates this? Looking at 
> Stackless' src/Python/bltinmodule.c:builtin_map, I see
>
>         for (i = 0; ; ++i) {
> 		...
>                 value = PyEval_CallObject(func, alist);
>                 ...
> 		PyList_SetItem(result, i, value)
> 		...
> 	}
>
> which looks recursive to me.

Ok, then I am mistaken.  I thought I had seen a commit message that 
said it was made non-recursive.  Either way, it's possible to write a 
non-recursive version of map or anything else, it's just a bit of work.

>> I am not sure about unicode encode/decode.  I don't think it's very 
>> common to do a tasklet switch in the middle of a unicode encode or 
>> decode, though :)
>
> I thought Stackless was not only about tasklet switching, but also
> about running without stack... Wouldn't it perform the "hard" switching
> if the stack is running too deep?

I don't think so, because the C stack should never run too deep, since 
most code that is going to recurse (from the python-view) will do it 
non-recursively (from the C-view).

>>> Py-func1() -> C-func1() -> Py-func2() -> switch to new tasklet
>>>               Py-func3() -> C-func2() -> throw exception()
>> I don't see how it could cause problems unless this is valid:
>>     Py_Func1 -> CPlusPlus_ExceptionCatcher -> Py_Func2 -> 
>> CPlusPlus_ExceptionThrower -> throws exception()
>
> It crashes in the process of throwing the exception - not because
> it finds no exception handler. It crashes because the internal
> data structures have been corrupted.
>
> I should have better made this example:
>
> Py-func1() -> C-func1() -> Py-func2() -> switch to new tasklet
>        Py-func3() -> Exception-Catcher -> C-func2() -> throw 
> exception()
>
>> If that *is* valid, how the heck does Py_Func2 get cleaned up?  I 
>> have to admit that I'm not very familiar with C++'s implementation of 
>> exceptions.
>
> Throwing exceptions through Python functions is not supported, indeed.
> However, I was trying to point out that the entire exception handling
> mechanism gets corrupted, even if there are valid handlers on the
> stack.

I don't think that the entire exception handling mechanism gets 
corrupted.  Each tasklet should always have a consistent view of the C 
stack.  Anyway, I don't think we need to discuss it any further unless 
someone writes a failing test... and if that is indeed the case, then 
the discussion should be taken to the stackless list.

-bob
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