[Cython] Hooking tp_clear()

[Greg Ewing]

Jeroen Demeyer wrote:

> I have a concrete use case where I want something like __dealloc__ but 
 > *before* Python attributes are cleared. So this really belongs in tp_clear().

Are you sure you can't do it in __del__?  From what I gather,
the presence of __del__ no longer prevents cyclic garbage
collection.

> I never really understood the technical difference between 
> tp_clear() and tp_dealloc(). It seems to me that these serve a very 
> similar purpose: why can't the garbage collector just call tp_dealloc()?

tp_dealloc is the inverse of tp_alloc -- its purpose is to
free the memory occupied by the object. This must not be done
until there are no more references to the object.

tp_clear is used to break reference cycles. After calling it,
there may still be references to the object from other objects
in the cycle, so tp_dealloc can't be done at that point.

-- 
Greg