[Cython] local variable handling in generators

[Stefan Behnel]

Stefan Behnel, 23.05.2011 11:15:
> Vitja Makarov, 23.05.2011 10:50:
>> 2011/5/23 Stefan Behnel:
>>> I'm fine with deallocating variables that are no longer used after the user
>>> explicitly assigned None to them (i.e. replace the None assignment by a
>>> simple "DECREF + set to NULL" in that case). I don't think we should be
>>> doing more than that.
>>
>> Hmm. Why should that be NULL if user sets it to None?
>
> Because there is no user visible difference. None will always be available,
> even if the Cython code no longer holds a reference to it. So changing "x =
> None" into "Py_DECREF(x); x=NULL" is just fine, as long as we can make sure
> 'x' is never accessed after this point.

The difference is clearer when I spell out the code for the first example, too:

     # x = None
     Py_INCREF(None)
     Py_DECREF(x)
     x = None

would be optimised into

     # x = None
     Py_DECREF(x)
     x = NULL

That's likely not a big difference, assuming that setting 'x' to None was 
worth it for the user, i.e. it will clean up the referenced object at that 
point. It may still be worth it inside of generators and on function 
return, which may now have one variable less to clean up or store away. A 
None value would still have to be DECREF-ed at least.

Stefan