Monkey patch __del__ to new function

del a deletes the name 'a' from the namespace, but not the object referenced by that name. See this:

>>> x = 7
>>> y = x
>>> del x
>>> print y
7

Also, some_object.__del__ is not guaranteed to be called at all.

Also, I already answered your question here (in german).

While special 'double underscore' methods like __del__, __str__, __repr__, etc. can be monkey-patched on the instance level, they'll just be ignored, unless they are called directly (e.g., if you take Omnifarious's answer: del a won't print a thing, but a.__del__() would).

If you still want to monkey patch a single instance a of class A at runtime, the solution is to dynamically create a class A1 which is derived from A, and then change a's class to the newly-created A1. Yes, this is possible, and a will behave as if nothing has changed - except that now it includes your monkey patched method.

Here's a solution based on a generic function I wrote for another question: Python method resolution mystery

def override(p, methods):
    oldType = type(p)
    newType = type(oldType.__name__ + "_Override", (oldType,), methods)
    p.__class__ = newType


class Test(object):
    def __str__(self):
        return "Test"

def p(self):
    print(str(self))

def monkey(x):
    override(x, {"__del__": p})

a=Test()
b=Test()
monkey(a)
print "Deleting a:"
del a
print "Deleting b:"
del b