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

Brian Quinlan brian@sweetapp.com
Fri, 18 Oct 2002 17:10:55 -0700

  1. Is there any reason why builtin methods cannot be proxied?

  2. It would be handy for my application if a callback could be triggered when an object has no more weak references attached to it.

It seems like my application could be a fairly common one:

C library pseudocode

def c_library_func(): # C code while 1: o = create_complex_object() user_call_back(o) del o

Python bindings pseudocode

def python_bindings_user_call_back (o): # C code py_o = create_python_wrapper_object(o) proxy = PyWeakref_NewProxy(py_o) python_function(py_o) Py_DECREF(proxy) Py_DECREF(py_o) # This will kill the proxy

Evil Python user code

evil = None def python_function(o): global evil o.foo() evil = o

start(python_function) evil.foo() # Nice exception because evil is a dead proxy

More evil Python user code

more_evil = None def python_function(o): global more_evil o.foo() more_evil = o.foo

start(python_function) more_evil() # Crash because the underlying data structures that # the Python wrapper object depends on are dead

My current solution to this problem is to create my own callable object type that supports weakrefs. That object is then used to wrap the real bound method object e.g.

def getattr(self, name): # This is C code callable = MyCallAble_New(Py_FindMethod(...); objects_to_kill_after_py_func_call.add(callable); return PyWeakref_NewProxy(callable);

Avoiding this hassle is the reason for my question.

Pruning callable objects that the user is done with is the reason for my request.

Cheers, Brian