Does `gc` treat Cython's `dealloc` similarly to `del`?

Question

Python's optional garbage collector gc ignores cycles that contain any object with a __del__ method:

Changed in version 3.4: Following PEP 442, objects with a __del__() method don’t end up in gc.garbage anymore.

Cython extension types can have a __dealloc__ method, but no __del__ method:

Note: There is no __del__() method for extension types.

For the purpose of collecting cycles, is the presence of __dealloc__ treated by the garbage collector as if a __del__ method is present? Or is __dealloc__ invisible to the garbage collector?

Starting from Python 3.4, objects with the method `__del__` [do not prevent reference cycles from being garbage collected](https://docs.python.org/3.6/library/weakref.html#comparing-finalizers-with-del-methods), so even if Cython classes had a `tp_del` slot (which they don't) *and* were on a reference cycle, they would still be garbage collected. So the quotation that mentions PEP 442 may have meant something else. — 0 _, May 04 '21 at 07:32

score 3 · Accepted Answer · edited May 04 '21 at 07:28

If you look at the generated C code you can see that Cython generates the destructor in the tp_dealloc slot, rather than the tp_del slot

cdef class B:
    def __dealloc__(self):
        pass

generates:

static PyTypeObject __pyx_type_5cy_gc_B = {
  PyVarObject_HEAD_INIT(0, 0)
  "cy_gc.B", /*tp_name*/
  sizeof(struct __pyx_obj_5cy_gc_B), /*tp_basicsize*/
  0, /*tp_itemsize*/
  __pyx_tp_dealloc_5cy_gc_B, /*tp_dealloc*/

  /* lines omitted */

  0, /*tp_del*/
  0, /*tp_version_tag*/
  #if PY_VERSION_HEX >= 0x030400a1
  0, /*tp_finalize*/
  #endif
};

You can easily verify that this is the case for other examples too (e.g. classes with automatically generated __dealloc__).

Therefore, for Python 3.4+:

Starting with Python 3.4, this list should be empty most of the time, except when using instances of C extension types with a non-NULL tp_del slot.

Cython classes should not end up in this list of uncollectable stuff since they don't have tp_del defined.

For earlier versions of Python I think you're also fine. Mostly because you still don't have a __del__ method, but also because cython automatically generates tp_traverse and tp_clear functions that should allow Python to break reference cycles involve Cython classes.

You can disable the generation of these tp_traverse and tp_clear functions. I'm a little unclear on what happens to objects that are in a reference cycle but don't have methods to detect it, or to break it. It's quite likely that they just continue to exist somewhere, but are inaccessible.

I think the concern (before Python 3.4) was that __del__ methods could make an object accessible again:

class C:
   def __del__(self):
      global x
      x = self

__dealloc__ is called after the point of no return, and so this isn't allowed (you just get a segmentation fault if you access x). Therefore they don't have to be stuck in gc.garbage in their indeterminate state.

Does `gc` treat Cython's `__dealloc__` similarly to `__del__`?

1 Answers1

Does `gc` treat Cython's `dealloc` similarly to `del`?