[Cython] 'with gil:' statement

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Thu Mar 17 08:40:25 CET 2011

On 03/17/2011 08:38 AM, Dag Sverre Seljebotn wrote:
> On 03/17/2011 12:24 AM, Greg Ewing wrote:
>> Stefan Behnel wrote:
>>> I'm not sure if this is a good idea. "nogil" blocks don't have a way 
>>> to handle exceptions, so simply jumping out of them because an inner 
>>> 'with gil' block raised an exception can have unexpected side effects.
>> Seems to me that the __Pyx_WriteUnraisable should be done at
>> the end of the 'with gil' block, and execution should then
>> continue from there.
>> In other words, the effect on exception handling should be
>> the same as if the 'with gil' block had been factored out into
>> a separate function having no exception return value.
> -1.
> I consider the fact that exceptions don't propagate from some 
> functions a "currently unfixable bug". We should plan for it being 
> fixed some day. Having a "with" statement alter execution flow in this 
> way is totally unintuitive to me.
> If you want this, it's better to introduce a new keyword like 
> "trywithgil: ... except:" (not that I'm in favour of that).
> We could perhaps fix exception propagation from nogil functions by 
> using some conventions + setjmp/longjmp. Mono does this when calling 
> into native code, and I recently did it manually in Cython to 
> propagate exceptions through the Fortran wrappers in SciPy. Also, the 
> GIL may not be around forever even in CPython? (All arguments I've 
> seen for keeping it has been along the lines of "it slows down serial 
> code", not that it is considered a good thing.)

Heh. I obviously meant that "removing it would slow down serial code".


> Designing a language around the GIL feels like a dead-end to me. I'm 
> OK with being practical in the face of the limitations of today; but 
> let's keep "with gil" and "with nogil" something that can become noops 
> in the future without too much pain. Yes, I know that if the GIL goes 
> it will break Stefan's lxml code, and I'm sure other code -- I'm just 
> saying that we shouldn't make the language design even more 
> GIL-centric than it already is.
> Anyway, I'm off to write some computational Fortran+OpenMP code 
> because dealing with threading and Python is just more than I can deal 
> with...
> Dag Sverre
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