"weakref" — Weak references
***************************

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

======================================================================

The "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.  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 "WeakKeyDictionary" and
"WeakValueDictionary" classes supplied by the "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 simply deleted.

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

"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 "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 "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 *file objects*, *generators*,
type objects, sockets, arrays, deques, regular expression pattern
objects, and code objects.

Changed in version 3.2: Added support for thread.lock, threading.Lock,
and code objects.

Several built-in types such as "list" and "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

**CPython implementation detail:** Other built-in types such as
"tuple" and "int" do not support weak references even when subclassed.

Extension types can easily be made to support weak references; see
Weak Reference Support.

class weakref.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 "None" to 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 "__del__()"
   method.

   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 "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.

   __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".

   Changed in version 3.4: Added the "__callback__" attribute.

weakref.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 *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 "ref()" function.

weakref.getweakrefcount(object)

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

weakref.getweakrefs(object)

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

class weakref.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.

"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.

WeakKeyDictionary.keyrefs()

   Return an iterable of the weak references to the keys.

class weakref.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.

"WeakValueDictionary" objects have an additional method that has the
same issues as the "keyrefs()" method of "WeakKeyDictionary" objects.

WeakValueDictionary.valuerefs()

   Return an iterable of the weak references to the values.

class weakref.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 weakref.WeakMethod(method)

   A custom "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.  "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()
      >>>

   New in version 3.4.

class weakref.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
   "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 "__del__()" method or a weak reference’s callback.

   When the program exits, each remaining live finalizer is called
   unless its "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 *interpreter shutdown* when module globals are liable to have
   been replaced by "None".

   __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
      "None".

   detach()

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

   peek()

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

   alive

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

   atexit

      A writable boolean property which by default is true.  When the
      program exits, it calls all remaining live finalizers for which
      "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*.

   New in version 3.4.

weakref.ReferenceType

   The type object for weak references objects.

weakref.ProxyType

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

weakref.CallableProxyType

   The type object for proxies of callable objects.

weakref.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.

See also:

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


Weak Reference Objects
======================

Weak reference objects have no methods and no attributes besides
"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
"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 "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.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


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.

   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]


Finalizer Objects
=================

The main benefit of using "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!")  
<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 "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()                                           
(<...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 "atexit" attribute to "False", a finalizer will be
called when the program exits if it is still alive.  For instance

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


Comparing finalizers with "__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 "remove()" method is called, or

* the program exits.

We might try to implement the class using a "__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, "__del__()" methods no longer prevent
reference cycles from being garbage collected, and module globals are
no longer forced to "None" during *interpreter shutdown*. So this code
should work without any issues on CPython.

However, handling of "__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
  "atexit.register()", "try: ... finally: ..." and "with: ..." do not
  guarantee that cleanup occurs either.
