A Descriptor class implementing weakref properties in Python
Add a property that acts like a normal attribute, but keeps a weak reference to anything that is assigned to it.
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 >>> obj.value
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
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