How to delete every reference of an object in Python?

Question

Supose you have something like:

x = "something"
b = x
l = [b]

How can you delete the object only having one reference, say x?

del x won't do the trick; the object is still reachable from b, for example.

Solution 1

No no no. Python has a garbage collector that has very strong territory issues - it won't mess with you creating objects, you don't mess with it deleting objects.

Simply put, it can't be done, and for a good reason.

If, for instance, your need comes from cases of, say, caching algorithms that keep references, but should not prevent data from being garbage collected once no one is using it, you might want to take a look at weakref.

Solution 2

The only solution I see right now is that you should make sure that you are holding the only reference to x, everyone else must not get x itself but a weak reference pointing to x. Weak references are implemented in the weakref module and you can use it this way:

>>> import weakref
>>> class TestClass(object):
...     def bark(self):
...         print "woof!"
...     def __del__(self):
...         print "destructor called"
...
>>> x = TestClass()
>>> b = weakref.proxy(x)
>>> b
<weakproxy at 0x7fa44dbddd08; to TestClass at 0x7fa44f9093d0>
>>> b.bark()
woof!
>>> del x
destructor called
>>> b.bark()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ReferenceError: weakly-referenced object no longer exists

However, note that not all classes can be weak-referenced. In particular, most built-in types cannot. Some built-in types can be weak-referenced if you subclass them (like dict), but others cannot (like int).

Solution 3

You don't. That's the entire point. Imagine if l is in a library outside of your control. It has every right to expect that the collection elements don't dissappear.

Also, imagine if it was otherwise. You'd have questions here on SO "How do I prevent others from deleting my objects?". As a language designer, you can't satisfy both demands.