weakref – Garbage-collectable references to objects

Purpose:Refer to an “expensive” object, but allow it to be garbage collected if there are no other non-weak references.
Available In:Since 2.1

The weakref module supports weak references to objects. A normal reference increments the reference count on the object and prevents it from being garbage collected. This is not always desirable, either when a circular reference might be present or when building a cache of objects that should be deleted when memory is needed.

References

Weak references to your objects are managed through the ref class. To retrieve the original object, call the reference object.

import weakref

class ExpensiveObject(object):
    def __del__(self):
        print '(Deleting %s)' % self

obj = ExpensiveObject()
r = weakref.ref(obj)

print 'obj:', obj
print 'ref:', r
print 'r():', r()

print 'deleting obj'
del obj
print 'r():', r()

In this case, since obj is deleted before the second call to the reference, the ref returns None.

$ python weakref_ref.py

obj: <__main__.ExpensiveObject object at 0x10046d410>
ref: <weakref at 0x100467838; to 'ExpensiveObject' at 0x10046d410>
r(): <__main__.ExpensiveObject object at 0x10046d410>
deleting obj
(Deleting <__main__.ExpensiveObject object at 0x10046d410>)
r(): None

Reference Callbacks

The ref constructor takes an optional second argument that should be a callback function to invoke when the referenced object is deleted.

import weakref

class ExpensiveObject(object):
    def __del__(self):
        print '(Deleting %s)' % self
        
def callback(reference):
    """Invoked when referenced object is deleted"""
    print 'callback(', reference, ')'

obj = ExpensiveObject()
r = weakref.ref(obj, callback)

print 'obj:', obj
print 'ref:', r
print 'r():', r()

print 'deleting obj'
del obj
print 'r():', r()

The callback receives the reference object as an argument, after the reference is “dead” and no longer refers to the original object. This lets you remove the weak reference object from a cache, for example.

$ python weakref_ref_callback.py

obj: <__main__.ExpensiveObject object at 0x10046c610>
ref: <weakref at 0x100468890; to 'ExpensiveObject' at 0x10046c610>
r(): <__main__.ExpensiveObject object at 0x10046c610>
deleting obj
callback( <weakref at 0x100468890; dead> )
(Deleting <__main__.ExpensiveObject object at 0x10046c610>)
r(): None

Proxies

Instead of using ref directly, it can be more convenient to use a proxy. Proxies can be used as though they were the original object, so you do not need to call the ref first to access the object.

import weakref

class ExpensiveObject(object):
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print '(Deleting %s)' % self

obj = ExpensiveObject('My Object')
r = weakref.ref(obj)
p = weakref.proxy(obj)

print 'via obj:', obj.name
print 'via ref:', r().name
print 'via proxy:', p.name
del obj
print 'via proxy:', p.name

If the proxy is access after the referent object is removed, a ReferenceError exception is raised.

$ python weakref_proxy.py

via obj: My Object
via ref: My Object
via proxy: My Object
(Deleting <__main__.ExpensiveObject object at 0x10046b490>)
via proxy:
Traceback (most recent call last):
  File "weakref_proxy.py", line 26, in <module>
    print 'via proxy:', p.name
ReferenceError: weakly-referenced object no longer exists

Cyclic References

One use for weak references is to allow cyclic references without preventing garbage collection. This example illustrates the difference between using regular objects and proxies when a graph includes a cycle.

First, we need a Graph class that accepts any object given to it as the “next” node in the sequence. For the sake of brevity, this Graph supports a single outgoing reference from each node, which results in boring graphs but makes it easy to create cycles. The function demo() is a utility function to exercise the graph class by creating a cycle and then removing various references.

import gc
from pprint import pprint
import weakref

class Graph(object):
    def __init__(self, name):
        self.name = name
        self.other = None
    def set_next(self, other):
        print '%s.set_next(%s (%s))' % (self.name, other, type(other))
        self.other = other
    def all_nodes(self):
        "Generate the nodes in the graph sequence."
        yield self
        n = self.other
        while n and n.name != self.name:
            yield n
            n = n.other
        if n is self:
            yield n
        return
    def __str__(self):
        return '->'.join([n.name for n in self.all_nodes()])
    def __repr__(self):
        return '%s(%s)' % (self.__class__.__name__, self.name)
    def __del__(self):
        print '(Deleting %s)' % self.name
        self.set_next(None)

class WeakGraph(Graph):
    def set_next(self, other):
        if other is not None:
            # See if we should replace the reference
            # to other with a weakref.
            if self in other.all_nodes():
                other = weakref.proxy(other)
        super(WeakGraph, self).set_next(other)
        return

def collect_and_show_garbage():
    "Show what garbage is present."
    print 'Collecting...'
    n = gc.collect()
    print 'Unreachable objects:', n
    print 'Garbage:', 
    pprint(gc.garbage)

def demo(graph_factory):
    print 'Set up graph:'
    one = graph_factory('one')
    two = graph_factory('two')
    three = graph_factory('three')
    one.set_next(two)
    two.set_next(three)
    three.set_next(one)
    
    print
    print 'Graphs:'
    print str(one)
    print str(two)
    print str(three)
    collect_and_show_garbage()

    print
    three = None
    two = None
    print 'After 2 references removed:'
    print str(one)
    collect_and_show_garbage()

    print
    print 'Removing last reference:'
    one = None
    collect_and_show_garbage()

Now we can set up a test program using the gc module to help us debug the leak. The DEBUG_LEAK flag causes gc to print information about objects that cannot be seen other than through the reference the garbage collector has to them.

import gc
from pprint import pprint
import weakref

from weakref_graph import Graph, demo, collect_and_show_garbage

gc.set_debug(gc.DEBUG_LEAK)

print 'Setting up the cycle'
print
demo(Graph)

print
print 'Breaking the cycle and cleaning up garbage'
print
gc.garbage[0].set_next(None)
while gc.garbage:
    del gc.garbage[0]
print
collect_and_show_garbage()

Even after deleting the local references to the Graph instances in demo(), the graphs all show up in the garbage list and cannot be collected. The dictionaries in the garbage list hold the attributes of the Graph instances. We can forcibly delete the graphs, since we know what they are:

$ python -u weakref_cycle.py

Setting up the cycle

Set up graph:
one.set_next(two (<class 'weakref_graph.Graph'>))
two.set_next(three (<class 'weakref_graph.Graph'>))
three.set_next(one->two->three (<class 'weakref_graph.Graph'>))

Graphs:
one->two->three->one
two->three->one->two
three->one->two->three
Collecting...
Unreachable objects: 0
Garbage:[]

After 2 references removed:
one->two->three->one
Collecting...
Unreachable objects: 0
Garbage:[]

Removing last reference:
Collecting...
gc: uncollectable <Graph 0x10046ff50>
gc: uncollectable <Graph 0x10046ff90>
gc: uncollectable <Graph 0x10046ffd0>
gc: uncollectable <dict 0x100363060>
gc: uncollectable <dict 0x100366460>
gc: uncollectable <dict 0x1003671f0>
Unreachable objects: 6
Garbage:[Graph(one),
 Graph(two),
 Graph(three),
 {'name': 'one', 'other': Graph(two)},
 {'name': 'two', 'other': Graph(three)},
 {'name': 'three', 'other': Graph(one)}]

Breaking the cycle and cleaning up garbage

one.set_next(None (<type 'NoneType'>))
(Deleting two)
two.set_next(None (<type 'NoneType'>))
(Deleting three)
three.set_next(None (<type 'NoneType'>))
(Deleting one)
one.set_next(None (<type 'NoneType'>))

Collecting...
Unreachable objects: 0
Garbage:[]

And now let’s define a more intelligent WeakGraph class that knows not to create cycles using regular references, but to use a ref when a cycle is detected.

import gc
from pprint import pprint
import weakref

from weakref_graph import Graph, demo

class WeakGraph(Graph):
    def set_next(self, other):
        if other is not None:
            # See if we should replace the reference
            # to other with a weakref.
            if self in other.all_nodes():
                other = weakref.proxy(other)
        super(WeakGraph, self).set_next(other)
        return
                
demo(WeakGraph)

Since the WeakGraph instances use proxies to refer to objects that have already been seen, as demo() removes all local references to the objects, the cycle is broken and the garbage collector can delete the objects for us.

$ python weakref_weakgraph.py

Set up graph:
one.set_next(two (<class '__main__.WeakGraph'>))
two.set_next(three (<class '__main__.WeakGraph'>))
three.set_next(one->two->three (<type 'weakproxy'>))

Graphs:
one->two->three
two->three->one->two
three->one->two->three
Collecting...
Unreachable objects: 0
Garbage:[]

After 2 references removed:
one->two->three
Collecting...
Unreachable objects: 0
Garbage:[]

Removing last reference:
(Deleting one)
one.set_next(None (<type 'NoneType'>))
(Deleting two)
two.set_next(None (<type 'NoneType'>))
(Deleting three)
three.set_next(None (<type 'NoneType'>))
Collecting...
Unreachable objects: 0
Garbage:[]

Caching Objects

The ref and proxy classes are considered “low level”. While they are useful for maintaining weak references to individual objects and allowing cycles to be garbage collected, if you need to create a cache of several objects the WeakKeyDictionary and WeakValueDictionary provide a more appropriate API.

As you might expect, the WeakValueDictionary uses weak references to the values it holds, allowing them to be garbage collected when other code is not actually using them.

To illustrate the difference between memory handling with a regular dictionary and WeakValueDictionary, let’s go experiment with explicitly calling the garbage collector again:

import gc
from pprint import pprint
import weakref

gc.set_debug(gc.DEBUG_LEAK)

class ExpensiveObject(object):
    def __init__(self, name):
        self.name = name
    def __repr__(self):
        return 'ExpensiveObject(%s)' % self.name
    def __del__(self):
        print '(Deleting %s)' % self
        
def demo(cache_factory):
    # hold objects so any weak references 
    # are not removed immediately
    all_refs = {}
    # the cache using the factory we're given
    print 'CACHE TYPE:', cache_factory
    cache = cache_factory()
    for name in [ 'one', 'two', 'three' ]:
        o = ExpensiveObject(name)
        cache[name] = o
        all_refs[name] = o
        del o # decref

    print 'all_refs =',
    pprint(all_refs)
    print 'Before, cache contains:', cache.keys()
    for name, value in cache.items():
        print '  %s = %s' % (name, value)
        del value # decref
        
    # Remove all references to our objects except the cache
    print 'Cleanup:'
    del all_refs
    gc.collect()

    print 'After, cache contains:', cache.keys()
    for name, value in cache.items():
        print '  %s = %s' % (name, value)
    print 'demo returning'
    return

demo(dict)
print
demo(weakref.WeakValueDictionary)

Notice that any loop variables that refer to the values we are caching must be cleared explicitly to decrement the reference count on the object. Otherwise the garbage collector would not remove the objects and they would remain in the cache. Similarly, the all_refs variable is used to hold references to prevent them from being garbage collected prematurely.

$ python weakref_valuedict.py

CACHE TYPE: <type 'dict'>
all_refs ={'one': ExpensiveObject(one),
 'three': ExpensiveObject(three),
 'two': ExpensiveObject(two)}
Before, cache contains: ['three', 'two', 'one']
  three = ExpensiveObject(three)
  two = ExpensiveObject(two)
  one = ExpensiveObject(one)
Cleanup:
After, cache contains: ['three', 'two', 'one']
  three = ExpensiveObject(three)
  two = ExpensiveObject(two)
  one = ExpensiveObject(one)
demo returning
(Deleting ExpensiveObject(three))
(Deleting ExpensiveObject(two))
(Deleting ExpensiveObject(one))

CACHE TYPE: weakref.WeakValueDictionary
all_refs ={'one': ExpensiveObject(one),
 'three': ExpensiveObject(three),
 'two': ExpensiveObject(two)}
Before, cache contains: ['three', 'two', 'one']
  three = ExpensiveObject(three)
  two = ExpensiveObject(two)
  one = ExpensiveObject(one)
Cleanup:
(Deleting ExpensiveObject(three))
(Deleting ExpensiveObject(two))
(Deleting ExpensiveObject(one))
After, cache contains: []
demo returning

The WeakKeyDictionary works similarly but uses weak references for the keys instead of the values in the dictionary.

The library documentation for weakref contains this warning:

Warning

Caution: Because a WeakValueDictionary is built on top of a Python dictionary, it must not change size when iterating over it. This can be difficult to ensure for a WeakValueDictionary because actions performed by the program during iteration may cause items in the dictionary to vanish “by magic” (as a side effect of garbage collection).

See also

weakref
Standard library documentation for this module.
gc
The gc module is the interface to the interpreter’s garbage collector.
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