Weak References in Python 2.1

Fred L. Drake, Jr.

PythonLabs @ Digital Creations

What are References?

• Pointers to values– Python name bindings– PyObject* variables in C extensions

Do References have Flavors?

• Strong References– All references in Python code– “Owned” PyObject* references in C

• Borrowed References– Un-owned PyObject* references in C

• Require great care when handled in C

• Typically have a short lifespan

So why Weak References?

• Variety is spice…

• Weak refs allow another way to interact with an object’s lifespan– Actions can be attached to object lifespan

without having to control it– Object can be shared among multiple users

without affecting lifespan

What do they look like?

>>> import weakref

>>> o = {}

>>> ref = weakref.ref(o)

>>> ref()

{}

>>> del o # no more dict!

>>> ref()

None

Can they be proxy objects?

>>> import weakref

>>> o = {}

>>> ref = weakref.proxy(o)

>>> ref.items()

[]

You didn’t “del o”…

>>> del o # no more dict!

>>> ref.items()

Traceback (most recent call last):

File "<stdin>", line 1

ReferenceError: ...

What else is there?

• WeakKeyDictionary– Similar to java.util.WeakHashMap– When value is only useful while key is live

• WeakValueDictionary– When value can be shared among unrelated

components but discarded when no one is using it

Isn’t that just caching?

• Yes!

• Clients no longer need to tell the cache it’s done with the object– Less obtrusive cache APIs lead to…– Cleaner client code

What can I use it with?

• Any class instance

• Extension types with explicit support– What other types do you want to use it with?

Tell me: fdrake@acm.org

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