Python 2.7 & 3.1 include a WeakSet class, a collection for storing objects using weak references (see the Python weakref module).

This project is a backport of the weakrefset module for Python 2.5 and 2.6. The tests require the unittest2 package.

You can install it with: pip install weakrefset

Usage is straightforward. You don't need to deal with weak references themselves, the WeakSet manages the references and you can add / remove / iterate over your objects directly:

    >>> from weakrefset import WeakSet
    >>> class Foo(object): pass
    ...
    >>> f1 = Foo()
    >>> f2 = Foo()
    >>> refs = WeakSet([f1, f2])
    >>> f3 = Foo()
    >>> refs.add(f3)
    >>> refs.remove(f1)
    >>> del f2
    >>> f3 in refs
    True
    >>> for f in refs:
    ...  print f
    ...
    <__main__.Foo object at 0x40e410>

To be compatible with Python 2.5, 2.6 and future versions of Python you can import it like this:

    try:
        # location in Python 2.7 and 3.1
        from weakref import WeakSet
    except ImportError:
        # separately installed
        from weakrefset import WeakSet