path: root/Doc/library/weakref.rst

:mod:`weakref` --- Weak references
==================================

.. module:: weakref
   :synopsis: Support for weak references and weak dictionaries.

.. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
.. moduleauthor:: Neil Schemenauer <nas@arctrix.com>
.. moduleauthor:: Martin von Löwis <martin@loewis.home.cs.tu-berlin.de>
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>

**Source code:** :source:`Lib/weakref.py`

--------------

The :mod:`weakref` module allows the Python programmer to create :dfn:`weak
references` to objects.

.. When making changes to the examples in this file, be sure to update
   Lib/test/test_weakref.py::libreftest too!

In the following, the term :dfn:`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,
:term:`garbage collection` is free to destroy the referent and reuse its memory
for something else.  However, until the object is actually destroyed the weak
reference may return the object even if there are no strong references to it.

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
:class:`WeakKeyDictionary` and :class:`WeakValueDictionary` classes supplied by
the :mod:`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
:class:`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
simply deleted.

:class:`WeakKeyDictionary` and :class:`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.  :class:`WeakSet` implements the :class:`set` interface,
but keeps weak references to its elements, just like a
:class:`WeakKeyDictionary` does.

:class:`finalize` provides a straight forward way to register a
cleanup function to be called when an object is garbage collected.
This is simpler to use than setting up a callback function on a raw
weak reference, since the module automatically ensures that the finalizer
remains alive until the object is collected.

Most programs should find that using one of these weak container types
or :class:`finalize` is all they need -- it's not usually necessary to
create your own weak references directly.  The low-level machinery is
exposed by the :mod:`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), instance methods, sets,
frozensets, some :term:`file objects <file object>`, :term:`generators <generator>`,
type objects, sockets, arrays, deques, regular expression pattern objects, and code
objects.

.. versionchanged:: 3.2
   Added support for thread.lock, threading.Lock, and code objects.

Several built-in types such as :class:`list` and :class:`dict` do not directly
support weak references but can add support through subclassing::

   class Dict(dict):
       pass

   obj = Dict(red=1, green=2, blue=3)   # this object is weak referenceable

.. impl-detail::

   Other built-in types such as :class:`tuple` and :class:`int` do not support weak
   references even when subclassed.

Extension types can easily be made to support weak references; see
:ref:`weakref-support`.


.. class:: ref(object[, callback])

   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 :const:`None` to be
   returned.  If *callback* is provided and not :const:`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 :meth:`__del__` method.

   Weak references are :term:`hashable` if the *object* is hashable.  They will
   maintain their hash value even after the *object* was deleted.  If
   :func:`hash` is called the first time only after the *object* was deleted,
   the call will raise :exc:`TypeError`.

   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.

   This is a subclassable type rather than a factory function.

   .. attribute:: __callback__

      This read-only attribute returns the callback currently associated to the
      weakref.  If there is no callback or if the referent of the weakref is
      no longer alive then this attribute will have value ``None``.

   .. versionchanged:: 3.4
      Added the :attr:`__callback__` attribute.


.. function:: proxy(object[, callback])

   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
   ``ProxyType`` or ``CallableProxyType``, depending on whether *object* is
   callable.  Proxy objects are not :term:`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 :func:`ref` function.


.. function:: getweakrefcount(object)

   Return the number of weak references and proxies which refer to *object*.


.. function:: getweakrefs(object)

   Return a list of all weak reference and proxy objects which refer to *object*.


.. class:: WeakKeyDictionary([dict])

   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.

   .. note::

      Caution: Because a :class:`WeakKeyDictionary` 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 :class:`WeakKeyDictionary` 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).

:class:`WeakKeyDictionary` objects have an additional method that
exposes 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
than needed.


.. method:: WeakKeyDictionary.keyrefs()

   Return an iterable of the weak references to the keys.


.. class:: WeakValueDictionary([dict])

   Mapping class that references values weakly.  Entries in the dictionary will be
   discarded when no strong reference to the value exists any more.

   .. note::

      Caution:  Because a :class:`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 :class:`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).

:class:`WeakValueDictionary` objects have an additional method that has the
same issues as the :meth:`keyrefs` method of :class:`WeakKeyDictionary`
objects.


.. method:: WeakValueDictionary.valuerefs()

   Return an iterable of the weak references to the values.


.. class:: WeakSet([elements])

   Set class that keeps weak references to its elements.  An element will be
   discarded when no strong reference to it exists any more.


.. class:: WeakMethod(method)

   A custom :class:`ref` subclass which simulates a weak reference to a bound
   method (i.e., a method defined on a class and looked up on an instance).
   Since a bound method is ephemeral, a standard weak reference cannot keep
   hold of it.  :class:`WeakMethod` has special code to recreate the bound
   method until either the object or the original function dies::

      >>> class C:
      ...     def method(self):
      ...         print("method called!")
      ...
      >>> c = C()
      >>> r = weakref.ref(c.method)
      >>> r()
      >>> r = weakref.WeakMethod(c.method)
      >>> r()
      <bound method C.method of <__main__.C object at 0x7fc859830220>>
      >>> r()()
      method called!
      >>> del c
      >>> gc.collect()
      0
      >>> r()
      >>>

   .. versionadded:: 3.4

.. class:: finalize(obj, func, *args, **kwargs)

   Return a callable finalizer object which will be called when *obj*
   is garbage collected. Unlike an ordinary weak reference, a finalizer
   will always survive until the reference object is collected, greatly
   simplifying lifecycle management.

   A finalizer is considered *alive* until it is called (either explicitly
   or at garbage collection), and after that it is *dead*.  Calling a live
   finalizer returns the result of evaluating ``func(*arg, **kwargs)``,
   whereas calling a dead finalizer returns :const:`None`.

   Exceptions raised by finalizer callbacks during garbage collection
   will be shown on the standard error output, but cannot be
   propagated.  They are handled in the same way as exceptions raised
   from an object's :meth:`__del__` method or a weak reference's
   callback.

   When the program exits, each remaining live finalizer is called
   unless its :attr:`atexit` attribute has been set to false.  They
   are called in reverse order of creation.

   A finalizer will never invoke its callback during the later part of
   the :term:`interpreter shutdown` when module globals are liable to have
   been replaced by :const:`None`.

   .. method:: __call__()

      If *self* is alive then mark it as dead and return the result of
      calling ``func(*args, **kwargs)``.  If *self* is dead then return
      :const:`None`.

   .. method:: detach()

      If *self* is alive then mark it as dead and return the tuple
      ``(obj, func, args, kwargs)``.  If *self* is dead then return
      :const:`None`.

   .. method:: peek()

      If *self* is alive then return the tuple ``(obj, func, args,
      kwargs)``.  If *self* is dead then return :const:`None`.

   .. attribute:: alive

      Property which is true if the finalizer is alive, false otherwise.

   .. attribute:: atexit

      A writable boolean property which by default is true.  When the
      program exits, it calls all remaining live finalizers for which
      :attr:`.atexit` is true.  They are called in reverse order of
      creation.

   .. note::

      It is important to ensure that *func*, *args* and *kwargs* do
      not own any references to *obj*, either directly or indirectly,
      since otherwise *obj* will never be garbage collected.  In
      particular, *func* should not be a bound method of *obj*.

   .. versionadded:: 3.4


.. data:: ReferenceType

   The type object for weak references objects.


.. data:: ProxyType

   The type object for proxies of objects which are not callable.


.. data:: CallableProxyType

   The type object for proxies of callable objects.


.. data:: ProxyTypes

   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:: ReferenceError

   Exception raised when a proxy object is used but the underlying object has been
   collected.  This is the same as the standard :exc:`ReferenceError` exception.


.. seealso::

   :pep:`205` - Weak References
      The proposal and rationale for this feature, including links to earlier
      implementations and information about similar features in other languages.


.. _weakref-objects:

Weak Reference Objects
----------------------

Weak reference objects have no methods and no attributes besides
:attr:`ref.__callback__`. A weak reference object allows 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
:const:`None`:

   >>> del o, o2
   >>> print(r())
   None

Testing that a weak reference object is still live should be done using the
expression ``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 :class:`ref` objects can be created through subclassing.
This is used in the implementation of the :class:`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 :class:`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.items():
               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


.. _weakref-example:

Example
-------

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.

.. Example contributed by Tim Peters.

::

   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]


.. _finalize-examples:

Finalizer Objects
-----------------

The main benefit of using :class:`finalize` is that it makes it simple
to register a callback without needing to preserve the returned finalizer
object.  For instance

    >>> import weakref
    >>> class Object:
    ...     pass
    ...
    >>> kenny = Object()
    >>> weakref.finalize(kenny, print, "You killed Kenny!")  #doctest:+ELLIPSIS
    <finalize object at ...; for 'Object' at ...>
    >>> del kenny
    You killed Kenny!

The finalizer can be called directly as well.  However the finalizer
will invoke the callback at most once.

    >>> def callback(x, y, z):
    ...     print("CALLBACK")
    ...     return x + y + z
    ...
    >>> obj = Object()
    >>> f = weakref.finalize(obj, callback, 1, 2, z=3)
    >>> assert f.alive
    >>> assert f() == 6
    CALLBACK
    >>> assert not f.alive
    >>> f()                     # callback not called because finalizer dead
    >>> del obj                 # callback not called because finalizer dead

You can unregister a finalizer using its :meth:`~finalize.detach`
method.  This kills the finalizer and returns the arguments passed to
the constructor when it was created.

    >>> obj = Object()
    >>> f = weakref.finalize(obj, callback, 1, 2, z=3)
    >>> f.detach()                                           #doctest:+ELLIPSIS
    (<...Object object ...>, <function callback ...>, (1, 2), {'z': 3})
    >>> newobj, func, args, kwargs = _
    >>> assert not f.alive
    >>> assert newobj is obj
    >>> assert func(*args, **kwargs) == 6
    CALLBACK

Unless you set the :attr:`~finalize.atexit` attribute to
:const:`False`, a finalizer will be called when the program exits if it
is still alive.  For instance

.. doctest::
   :options: +SKIP

   >>> obj = Object()
   >>> weakref.finalize(obj, print, "obj dead or exiting")
   <finalize object at ...; for 'Object' at ...>
   >>> exit()
   obj dead or exiting


Comparing finalizers with :meth:`__del__` methods
-------------------------------------------------

Suppose we want to create a class whose instances represent temporary
directories.  The directories should be deleted with their contents
when the first of the following events occurs:

* the object is garbage collected,
* the object's :meth:`remove` method is called, or
* the program exits.

We might try to implement the class using a :meth:`__del__` method as
follows::

    class TempDir:
        def __init__(self):
            self.name = tempfile.mkdtemp()

        def remove(self):
            if self.name is not None:
                shutil.rmtree(self.name)
                self.name = None

        @property
        def removed(self):
            return self.name is None

        def __del__(self):
            self.remove()

Starting with Python 3.4, :meth:`__del__` methods no longer prevent
reference cycles from being garbage collected, and module globals are
no longer forced to :const:`None` during :term:`interpreter shutdown`.
So this code should work without any issues on CPython.

However, handling of :meth:`__del__` methods is notoriously implementation
specific, since it depends on internal details of the interpreter's garbage
collector implementation.

A more robust alternative can be to define a finalizer which only references
the specific functions and objects that it needs, rather than having access
to the full state of the object::

    class TempDir:
        def __init__(self):
            self.name = tempfile.mkdtemp()
            self._finalizer = weakref.finalize(self, shutil.rmtree, self.name)

        def remove(self):
            self._finalizer()

        @property
        def removed(self):
            return not self._finalizer.alive

Defined like this, our finalizer only receives a reference to the details
it needs to clean up the directory appropriately. If the object never gets
garbage collected the finalizer will still be called at exit.

The other advantage of weakref based finalizers is that they can be used to
register finalizers for classes where the definition is controlled by a
third party, such as running code when a module is unloaded::

    import weakref, sys
    def unloading_module():
        # implicit reference to the module globals from the function body
    weakref.finalize(sys.modules[__name__], unloading_module)


.. note::

   If you create a finalizer object in a daemonic thread just as the program
   exits then there is the possibility that the finalizer
   does not get called at exit.  However, in a daemonic thread
   :func:`atexit.register`, ``try: ... finally: ...`` and ``with: ...``
   do not guarantee that cleanup occurs either.