[Cython] 'with gil:' statement

Stefan Behnel stefan_ml at behnel.de
Fri Mar 18 11:10:46 CET 2011

mark florisson, 18.03.2011 10:52:
> On 18 March 2011 07:07, Stefan Behnel wrote:
>> Greg Ewing, 18.03.2011 01:18:
>>> mark florisson wrote:
>>>> I think we could support it without having to acquire
>>>> the GIL in the finally clause.
>>> That was the intention -- the code in the finally clause would
>>> be subject to the same nogil restrictions as the rest of
>>> the nogil block.
>>> My point is that as long as you're allowing exceptions to be
>>> tunnelled through nogil blocks, they should respect any finally
>>> clauses that they pass through on the way.
>> +1
> Ok, I will give it a go and try to allow it when they surround with
> gil blocks. I would however like to reiterate that it is a
> special-case, inconsistent with previous behaviour, and basically
> extends the language and won't work for functions that are called and
> declared 'with gil'. But it is convenient, so I can't help but like it
> at the same time :]

I'm not sure I understand why you think it's so bad, and why it would be 
inconsistent with previous behaviour.

The only real problem I see is that you could do things like this:

     with nogil:
             with gil: raise ...
             with gil: raise ...

i.e. you could loose the original exception. Even worse:

     with nogil:
             with gil: raise ...
             with gil:
                 try: raise
                 except: pass

Here, it must be made sure that the original exception still gets raised 
properly at the end. That's a problem in Py2, where exceptions are badly 
scoped, i.e. Python code that runs in the interpreter could fail to reset 
the original exception after catching another one. But I guess these things 
are up to the "with gil" block/function, rather than the above "finally" 

Actually, I think I still find it more convenient to not provide *any* 
special exception paths through nogil code, i.e. to not let exceptions in 
"with gil" blocks exit from outer "nogil" blocks. That would avoid all of 
the semantic difficulties above.


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