[Python-Dev] Weakref design questions (original) (raw)

Brian Quinlan brian@sweetapp.com
Fri, 18 Oct 2002 19:51:18 -0700

David Abrahams:

> So I proxy the bound methods and kill them when p dies.

I don't quite understand what "proxy the bound methods" means.

Most extension types have a function like this:

PyObject * MyObject_getattr(MyObject* self, char * name) {
/* check for attributes */ ...

  /* ok, not an attribute, now check the methods
return Py_FindMethod(
        MyObject_methods, 
        (PyObject *) self, 
        name);

}

Py_FindMethod() returns a PyCFunctionObject. The PyCFunctionObject will own a reference to the MyObject "self".

Since "self" has a limited lifetime this would be bad. So we could do this:

PyObject * MyObject_getattr(MyObject* self, char * name) {
/* check for attributes */ ...

  /* ok, not an attribute, now check the methods
method = Py_FindMethod(
        MyObject_methods, 
        (PyObject *) self, 
        name);

if (method != NULL) {
    add_to_list_of_objects_to_kill(method)
    return PyWeakref_NewProxy(method);
}

return NULL;

}

But we can't quite do this because builtin functions are not proxyable.

So you want a weakrefref. Well, I think this problem can be solved by applying the Fundamental Theorem of Software Engineering: apply an extra level of indirection.

That's what I do. I was just wondering if there is any reason not to make C functions weakref/proxyable.

But Guido's approach of having the wrapper methods check for a null DOM* seems like a reasonable one to me.

It's not unreasonable and it is a bit simpler. But it is more work and reduces performance slightly in the case where no check need be performed i.e. the object is owned by Python.

That's clear to me now. Very interesting thread; thanks for posting it here!

Yeah, my first non-stupid post to python-dev.

Cheers, Brian