Jeroen Demeyer schrieb am 06.09.2018 um 22:54:

Cython's __dealloc__ special method is meant to deal with cleaning up instances of cdef classes. However, this hooks tp_dealloc() and does not have meaningful access to Python attributes, since those might have been cleared by tp_clear().

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().

Using a PyObject* attribute in the cdef class with manual reference counting is not a solution since this attribute could genuinely occur in a reference cycle.

So I would suggest to support a __clear__ special method, which would then be called both by tp_clear() and tp_dealloc(). It's important to note that this should be idempotent: it will be called at least once before Python attributes are cleared but it may also be called later.

Maybe you actually want "tp_finalize"? https://www.python.org/dev/peps/pep-0442/ Cython moves "__del__" methods there in Py3.4+.

PS: 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()?

The problem are reference cycles, in which there definitely is a life reference to the object *somewhere* else. Thus, the GC cannot simply deallocate the object, it must try to delete the references instead. This is what "tp_clear" is used for, it clears all references that an object inside of a reference cycle has towards other objects (or at least those that can participate in that cycle). This will (hopefully) trigger a cascade of deallocations along the cycle. If that isn't enough, and there is still a cycle, then the clearing needs to be repeated until all references to the last object in the cycle are cleared. AFAIR, tp_clear() is *only* called by the cyclic garbage collector and not during normal refcounting deallocation. The GC process is: tp_visit() to detect cycles, tp_clear() to break them. tp_dealloc() is then only called indirectly by the normal refcounting cleanup, not directly by the GC. Stefan