Delete an instance from its class' dict in desctructor?

Question

I'm trying to create a class that saves all of its instances in a dictionary:

>>> class X:
    def __new__(cls, index):
        if index in cls._instances:
            return cls._instances[index]
        self = object.__new__(cls)
        self.index = index
        cls._instances[index] = self
        return self
    def __del__(self):
        del type(self)._instances[self.index]
    _instances = {}

However, the __del__ doesn't seem to work:

>>> x = X(1)
>>> del x
>>> X._instances
{1: <__main__.X object at 0x00000000035166D8>}
>>> 

What am I doing wrong?

Answer 1

Building on Kirk Strauser's answer, I'd like to point out that, when you del x , the class' _instances still holds another reference to x - and thus it can't be garbage collected (and __del__ won't run.

Instead of doing this kind of low-level magic, you probably should be using weakref s , which were implemented especially for this purpose.

WeakValueDictinary , in particular, suits your needs perfectly, and you can fill it on __init__ instead of fiddling with __new__ and __del__

Answer 2

You're not doing anything wrong, but __del__ isn't quite what you think. From the docs on it :

Note del x doesn't directly call x.__del__() — the former decrements the reference count for x by one, and the latter is only called when x 's reference count reaches zero.

Running this from the interpreter is particularly tricky because command history or other mechanisms may hold references to x for an indeterminate amount of time.

By the way, your code looks an awful lot like a defaultdict with X as the factory. It may be more straightforward to use something like that to be more explicit (ergo more Pythonic) about what you're trying to do.