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weakref.WeakSet

weak collections

A set that holds its objects through weak references: a deleted object drops out on its own. Handy for a registry of live instances or subscribers.

Syntax

s = weakref.WeakSet()

The classic example is tracking created instances of a class: a plain set in a class attribute would keep them forever and leak memory. Elements must support weak references, so numbers and strings cannot go into a WeakSet. As with WeakKeyDictionary, the size can shrink at any moment, so len() is a snapshot, not a guarantee.

Examples

import weakref
class Player:
    online = weakref.WeakSet()
    def __init__(self):
        Player.online.add(self)
a, b = Player(), Player()
print(len(Player.online))  # → 2
del a
print(len(Player.online))  # → 1

See also

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