weakref — Weak references
New in version 2.1.
Source code: Lib/weakref.py
weakref module allows the Python programmer to create weak
references to objects.
In the following, the term referent means the object which is referred to by a weak reference.
A weak reference to an object is not enough to keep the object alive: when the only remaining references to a referent are weak references, garbage collection is free to destroy the referent and reuse its memory for something else. A primary use for weak references is to implement caches or mappings holding large objects, where it’s desired that a large object not be kept alive solely because it appears in a cache or mapping.
For example, if you have a number of large binary image objects, you may wish to
associate a name with each. If you used a Python dictionary to map names to
images, or images to names, the image objects would remain alive just because
they appeared as values or keys in the dictionaries. The
WeakValueDictionary classes supplied by
weakref module are an alternative, using weak references to construct
mappings that don’t keep objects alive solely because they appear in the mapping
objects. If, for example, an image object is a value in a
WeakValueDictionary, then when the last remaining references to that
image object are the weak references held by weak mappings, garbage collection
can reclaim the object, and its corresponding entries in weak mappings are
WeakValueDictionary use weak references
in their implementation, setting up callback functions on the weak references
that notify the weak dictionaries when a key or value has been reclaimed by
garbage collection. Most programs should find that using one of these weak
dictionary types is all they need – it’s not usually necessary to create your
own weak references directly. The low-level machinery used by the weak
dictionary implementations is exposed by the
weakref module for the
benefit of advanced uses.
Not all objects can be weakly referenced; those objects which can include class
instances, functions written in Python (but not in C), methods (both bound and
unbound), sets, frozensets, file objects, generators, type objects,
DBcursor objects from the
bsddb module, sockets, arrays, deques,
regular expression pattern objects, and code objects.
Changed in version 2.4: Added support for files, sockets, arrays, and patterns.
Changed in version 2.7: Added support for thread.lock, threading.Lock, and code objects.
class Dict(dict): pass obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
Extension types can easily be made to support weak references; see Weak Reference Support.
Return a weak reference to object. The original object can be retrieved by calling the reference object if the referent is still alive; if the referent is no longer alive, calling the reference object will cause
Noneto be returned. If callback is provided and not
None, and the returned weakref object is still alive, the callback will be called when the object is about to be finalized; the weak reference object will be passed as the only parameter to the callback; the referent will no longer be available.
It is allowable for many weak references to be constructed for the same object. Callbacks registered for each weak reference will be called from the most recently registered callback to the oldest registered callback.
Exceptions raised by the callback will be noted on the standard error output, but cannot be propagated; they are handled in exactly the same way as exceptions raised from an object’s
Weak references are hashable if the object is hashable. They will maintain their hash value even after the object was deleted. If
hash()is called the first time only after the object was deleted, the call will raise
Weak references support tests for equality, but not ordering. If the referents are still alive, two references have the same equality relationship as their referents (regardless of the callback). If either referent has been deleted, the references are equal only if the reference objects are the same object.
Changed in version 2.4: This is now a subclassable type rather than a factory function; it derives from
Return a proxy to object which uses a weak reference. This supports use of the proxy in most contexts instead of requiring the explicit dereferencing used with weak reference objects. The returned object will have a type of either
CallableProxyType, depending on whether object is callable. Proxy objects are not hashable regardless of the referent; this avoids a number of problems related to their fundamentally mutable nature, and prevent their use as dictionary keys. callback is the same as the parameter of the same name to the
Return the number of weak references and proxies which refer to object.
Return a list of all weak reference and proxy objects which refer to object.
Mapping class that references keys weakly. Entries in the dictionary will be discarded when there is no longer a strong reference to the key. This can be used to associate additional data with an object owned by other parts of an application without adding attributes to those objects. This can be especially useful with objects that override attribute accesses.
Caution: Because a
WeakKeyDictionaryis built on top of a Python dictionary, it must not change size when iterating over it. This can be difficult to ensure for a
WeakKeyDictionarybecause actions performed by the program during iteration may cause items in the dictionary to vanish “by magic” (as a side effect of garbage collection).
WeakKeyDictionary objects have the following additional methods. These
expose the internal references directly. The references are not guaranteed to
be “live” at the time they are used, so the result of calling the references
needs to be checked before being used. This can be used to avoid creating
references that will cause the garbage collector to keep the keys around longer
Return an iterable of the weak references to the keys.
New in version 2.5.
Return a list of weak references to the keys.
New in version 2.5.
Mapping class that references values weakly. Entries in the dictionary will be discarded when no strong reference to the value exists any more.
Caution: Because a
WeakValueDictionaryis built on top of a Python dictionary, it must not change size when iterating over it. This can be difficult to ensure for a
WeakValueDictionarybecause actions performed by the program during iteration may cause items in the dictionary to vanish “by magic” (as a side effect of garbage collection).
Return an iterable of the weak references to the values.
New in version 2.5.
Return a list of weak references to the values.
New in version 2.5.
Set class that keeps weak references to its elements. An element will be discarded when no strong reference to it exists any more.
New in version 2.7.
The type object for weak references objects.
The type object for proxies of objects which are not callable.
The type object for proxies of callable objects.
Sequence containing all the type objects for proxies. This can make it simpler to test if an object is a proxy without being dependent on naming both proxy types.
Exception raised when a proxy object is used but the underlying object has been collected. This is the same as the standard
- PEP 205 - Weak References
- The proposal and rationale for this feature, including links to earlier implementations and information about similar features in other languages.
8.11.1. Weak Reference Objects
Weak reference objects have no attributes or methods, but do allow the referent to be obtained, if it still exists, by calling it:
>>> import weakref >>> class Object: ... pass ... >>> o = Object() >>> r = weakref.ref(o) >>> o2 = r() >>> o is o2 True
If the referent no longer exists, calling the reference object returns
>>> del o, o2 >>> print r() None
Testing that a weak reference object is still live should be done using the
ref() is not None. Normally, application code that needs to use
a reference object should follow this pattern:
# r is a weak reference object o = r() if o is None: # referent has been garbage collected print "Object has been deallocated; can't frobnicate." else: print "Object is still live!" o.do_something_useful()
Using a separate test for “liveness” creates race conditions in threaded applications; another thread can cause a weak reference to become invalidated before the weak reference is called; the idiom shown above is safe in threaded applications as well as single-threaded applications.
Specialized versions of
ref objects can be created through subclassing.
This is used in the implementation of the
WeakValueDictionary to reduce
the memory overhead for each entry in the mapping. This may be most useful to
associate additional information with a reference, but could also be used to
insert additional processing on calls to retrieve the referent.
This example shows how a subclass of
ref can be used to store
additional information about an object and affect the value that’s returned when
the referent is accessed:
import weakref class ExtendedRef(weakref.ref): def __init__(self, ob, callback=None, **annotations): super(ExtendedRef, self).__init__(ob, callback) self.__counter = 0 for k, v in annotations.iteritems(): setattr(self, k, v) def __call__(self): """Return a pair containing the referent and the number of times the reference has been called. """ ob = super(ExtendedRef, self).__call__() if ob is not None: self.__counter += 1 ob = (ob, self.__counter) return ob
This simple example shows how an application can use object IDs to retrieve objects that it has seen before. The IDs of the objects can then be used in other data structures without forcing the objects to remain alive, but the objects can still be retrieved by ID if they do.
import weakref _id2obj_dict = weakref.WeakValueDictionary() def remember(obj): oid = id(obj) _id2obj_dict[oid] = obj return oid def id2obj(oid): return _id2obj_dict[oid]