Weak References in python

Question

I have been trying to understand how python weak reference lists/dictionaries work. I've read the documentation for it, however I cannot figure out how they work, and what they can be used for. Could anyone give me a basic example of what they do and an explanation of how they work?

(EDIT) Using Thomas's code, when i substitute obj for [1,2,3] it throws:

Traceback (most recent call last): File "C:/Users/nonya/Desktop/test.py", line 9, in <module> r = weakref.ref(obj) TypeError: cannot create weak reference to 'list' object 

Answer 1

Theory

The reference count usually works as such: each time you create a reference to an object, it is increased by one, and whenever you delete a reference, it is decreased by one.

Weak references allow you to create references to an object that will not increase the reference count.

The reference count is used by python's Garbage Collector when it runs: any object whose reference count is 0 will be garbage collected.

You would use weak references for expensive objects, or to avoid circle references (although the garbage collector usually does it on its own).

Usage

Here's a working example demonstrating their usage:

import weakref import gc  class MyObject(object):     def my_method(self):         print 'my_method was called!'  obj = MyObject() r = weakref.ref(obj)  gc.collect() assert r() is obj #r() allows you to access the object referenced: it's there.  obj = 1 #Let's change what obj references to gc.collect() assert r() is None #There is no object left: it was gc'ed. 

Answer 2

Just want to point out that weakref.ref does not work for built-in list because there is no __weakref__ in the __slots__ of list. For example, the following code defines a list container that supports weakref.

import weakref  class weaklist(list):     __slots__ = ('__weakref__',)  l = weaklist() r = weakref.ref(l)