A Descriptor class implementing weakref properties in Python 2/3

Add a property that acts like a normal attribute, but keeps a weak reference to anything that is assigned to it.

Example:

First let's define WeakValue using WeakProperty and SomeClass that will be weakly referenced:

>>> import weakref_property
>>> import gc

>>> class WeakValue(object):
...     value = weakref_property.WeakProperty('value')

>>> class SomeClass(object):
...     pass

You can assign and retrieve a value:

>>> a = SomeClass()
>>> obj = WeakValue()
>>> obj.value = a
>>> obj.value  # doctest: +ELLIPSIS
<__main__.SomeClass object at ...>

But it is kept as a weakref, so it can get collected. If it is collected, the property returns None:

>>> del a
>>> gc.collect()  # force gc to collect `a` object
0
>>> print(obj.value)
None

You can also delete the value completelly:

>>> del obj.value
>>> obj.value
Traceback (most recent call last):
    ...
AttributeError: 'WeakValue' object has no attribute 'value'

Sadly, weakrefs do not work on tuples, ints and not-subclassed lists and dicts:

>>> obj.value = []
Traceback (most recent call last):
    ...
TypeError: cannot create weak reference to 'list' object