[Python-Dev] Stackless Python
Bob Ippolito
bob at redivi.com
Mon May 31 17:39:01 EDT 2004
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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