Python weakref Module

The weakref module in Python provides functions for creating weak references to objects. Weak references allow the referenced object to be garbage collected when there are no strong references to it, which can help with memory management in certain scenarios.

Table of Contents

1. Introduction
2. Weak Reference Types
   • ref
   • proxy
3. Weak Reference Methods
   • callback
4. Weak Reference Collections
   • WeakKeyDictionary
   • WeakValueDictionary
   • WeakSet
5. Examples
   • Creating Weak References
   • Using Callbacks with Weak References
   • Using Weak Reference Collections
6. Conclusion
   7References

Introduction

The weakref module provides support for weak references in Python. A weak reference allows an object to be referenced without preventing it from being garbage collected. This is useful for caching, observing, and other scenarios where you want to avoid creating reference cycles that would prevent garbage collection.

Weak Reference Types

ref

The ref function creates a weak reference to an object. The referenced object can be retrieved using the weak reference.

import weakref

class MyClass:
    pass

obj = MyClass()
weak_ref = weakref.ref(obj)
print(weak_ref)
print(weak_ref())

Output:

<weakref at 0x0000025235A427F0; to 'MyClass' at 0x0000025235A35CD0>
<__main__.MyClass object at 0x0000025235A35CD0>

proxy

The proxy function creates a proxy object that behaves like a weak reference but automatically dereferences itself.

import weakref

class MyClass:
    def __init__(self, value):
        self.value = value

obj = MyClass(10)
weak_proxy = weakref.proxy(obj)
print(weak_proxy.value)

Output:

10

Weak Reference Methods

callback

A callback function can be registered to be called when the referenced object is about to be finalized.

import weakref

class MyClass:
    pass

def on_finalize(ref):
    print("Object is being garbage collected")

obj = MyClass()
weak_ref = weakref.ref(obj, on_finalize)
del obj

Output:

Object is being garbage collected

Weak Reference Collections

WeakKeyDictionary

A WeakKeyDictionary is a dictionary that holds weak references to its keys. When a key is garbage collected, the corresponding item is automatically removed from the dictionary.

import weakref

class MyClass:
    pass

obj = MyClass()
weak_dict = weakref.WeakKeyDictionary()
weak_dict[obj] = "value"
print(list(weak_dict.keys()))
del obj
print(list(weak_dict.keys()))

Output:

[<__main__.MyClass object at 0x0000016866255E50>]
[]

WeakValueDictionary

A WeakValueDictionary is a dictionary that holds weak references to its values. When a value is garbage collected, the corresponding item is automatically removed from the dictionary.

import weakref

class MyClass:
    pass

obj = MyClass()
weak_dict = weakref.WeakValueDictionary()
weak_dict["key"] = obj
print(list(weak_dict.values()))
del obj
print(list(weak_dict.values()))

Output:

[<__main__.MyClass object at 0x000002511F655E50>]
[]

WeakSet

A WeakSet is a set that holds weak references to its elements. When an element is garbage collected, it is automatically removed from the set.

import weakref

class MyClass:
    pass

obj = MyClass()
weak_set = weakref.WeakSet()
weak_set.add(obj)
print(list(weak_set))
del obj
print(list(weak_set))

Output:

[<__main__.MyClass object at 0x000002B5C7145E20>]
[]

Examples

Creating Weak References

Create a weak reference to an object and retrieve it.

import weakref

class MyClass:
    pass

obj = MyClass()
weak_ref = weakref.ref(obj)
print(weak_ref())  # Retrieve the referenced object

Output:

<__main__.MyClass object at 0x000001C61FF45C70>

Using Callbacks with Weak References

Register a callback to be called when the referenced object is about to be finalized.

import weakref

class MyClass:
    pass

def on_finalize(ref):
    print("Object is being garbage collected")

obj = MyClass()
weak_ref = weakref.ref(obj, on_finalize)
del obj  # Trigger garbage collection

Output:

Object is being garbage collected

Using Weak Reference Collections

Use WeakKeyDictionary to manage objects with weak references.

import weakref

class MyClass:
    pass

obj = MyClass()
weak_dict = weakref.WeakKeyDictionary()
weak_dict[obj] = "value"
print(list(weak_dict.keys()))
del obj
print(list(weak_dict.keys()))

Output:

[<__main__.MyClass object at 0x0000021C86375E50>]
[]

Conclusion

The weakref module in Python provides functions for managing references to objects without preventing their garbage collection. This is especially useful in scenarios like caching, observing, and managing reference cycles, helping to optimize memory usage and program performance.

References

• Python weakref module documentation