Doc/lib/libweakref.tex

   1 \section{\module{weakref} ---
   2          Weak references}
   3
   4 \declaremodule{extension}{weakref}
   5 \modulesynopsis{Support for weak references and weak dictionaries.}
   6 \moduleauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
   7 \moduleauthor{Neil Schemenauer}{nas@arctrix.com}
   8 \moduleauthor{Martin von L\"owis}{martin@loewis.home.cs.tu-berlin.de}
   9 \sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
  10
  11 \versionadded{2.1}
  12
  13
  14 The \module{weakref} module allows the Python programmer to create
  15 \dfn{weak references} to objects.
  16
  17 In the discussion which follows, the term \dfn{referent} means the
  18 object which is referred to by a weak reference.
  19
  20 XXX --- need to say more here!
  21
  22 Not all objects can be weakly referenced; those objects which do
  23 include class instances, functions written in Python (but not in C),
  24 and methods (both bound and unbound).  Extension types can easily
  25 be made to support weak references; see section \ref{weakref-extension},
  26 ``Weak References in Extension Types,'' for more information.
  27
  28
  29 \begin{funcdesc}{ref}{object\optional{, callback}}
  30   Return a weak reference to \var{object}.  The original object can be
  31   retrieved by calling the reference object if the referent is still
  32   alive; if the referent is no longer alive, calling the reference
  33   object will cause \code{None} to be returned.  If \var{callback} is
  34   provided, it will be called when the object is about to be
  35   finalized; the weak reference object will be passed as the only
  36   parameter to the callback; the referent will no longer be available.
  37
  38   It is allowable for many weak references to be constructed for the
  39   same object.  Callbacks registered for each weak reference will be
  40   called from the most recently registered callback to the oldest
  41   registered callback.
  42
  43   Exceptions raised by the callback will be noted on the standard
  44   error output, but cannot be propagated; they are handled in exactly
  45   the same way as exceptions raised from an object's
  46   \method{__del__()} method.
  47
  48   Weak references are hashable if the \var{object} is hashable.  They
  49   will maintain their hash value even after the \var{object} was
  50   deleted.  If \function{hash()} is called the first time only after
  51   the \var{object} was deleted, the call will raise
  52   \exception{TypeError}.
  53
  54   Weak references support tests for equality, but not ordering.  If
  55   the referents are still alive, two references have the same
  56   equality relationship as their referents (regardless of the
  57   \var{callback}).  If either referent has been deleted, the
  58   references are equal only if the reference objects are the same
  59   object.
  60 \end{funcdesc}
  61
  62 \begin{funcdesc}{proxy}{object\optional{, callback}}
  63   Return a proxy to \var{object} which uses a weak reference.  This
  64   supports use of the proxy in most contexts instead of requiring the
  65   explicit dereferencing used with weak reference objects.  The
  66   returned object will have a type of either \code{ProxyType} or
  67   \code{CallableProxyType}, depending on whether \var{object} is
  68   callable.  Proxy objects are not hashable regardless of the
  69   referent; this avoids a number of problems related to their
  70   fundamentally mutable nature, and prevent their use as dictionary
  71   keys.  \var{callback} is the same as the parameter of the same name
  72   to the \function{ref()} function.
  73 \end{funcdesc}
  74
  75 \begin{funcdesc}{getweakrefcount}{object}
  76   Return the number of weak references and proxies which refer to
  77   \var{object}.
  78 \end{funcdesc}
  79
  80 \begin{funcdesc}{getweakrefs}{object}
  81   Return a list of all weak reference and proxy objects which refer to
  82   \var{object}.
  83 \end{funcdesc}
  84
  85 \begin{classdesc}{WeakKeyDictionary}{\optional{dict}}
  86   Mapping class that references keys weakly.  Entries in the
  87   dictionary will be discarded when there is no longer a strong
  88   reference to the key.  This can be used to associate additional data
  89   with an object owned by other parts of an application without adding
  90   attributes to those objects.  This can be especially useful with
  91   objects that override attribute accesses.
  92 \end{classdesc}
  93
  94 \begin{classdesc}{WeakValueDictionary}{\optional{dict}}
  95   Mapping class that references values weakly.  Entries in the
  96   dictionary will be discarded when no strong reference to the value
  97   exists any more.
  98 \end{classdesc}
  99
 100 \begin{datadesc}{ReferenceType}
 101   The type object for weak references objects.
 102 \end{datadesc}
 103
 104 \begin{datadesc}{ProxyType}
 105   The type object for proxies of objects which are not callable.
 106 \end{datadesc}
 107
 108 \begin{datadesc}{CallableProxyType}
 109   The type object for proxies of callable objects.
 110 \end{datadesc}
 111
 112 \begin{datadesc}{ProxyTypes}
 113   Sequence containing all the type objects for proxies.  This can make
 114   it simpler to test if an object is a proxy without being dependent
 115   on naming both proxy types.
 116 \end{datadesc}
 117
 118 \begin{excdesc}{ReferenceError}
 119   Exception raised when a proxy object is used but the underlying
 120   object has been collected.  This is the same as the standard
 121   \exception{ReferenceError} exception.
 122 \end{excdesc}
 123
 124
 125 \begin{seealso}
 126   \seepep{0205}{Weak References}{The proposal and rationale for this
 127                 feature, including links to earlier implementations
 128                 and information about similar features in other
 129                 languages.}
 130 \end{seealso}
 131
 132
 133 \subsection{Weak Reference Objects
 134             \label{weakref-objects}}
 135
 136 Weak reference objects have no attributes or methods, but do allow the
 137 referent to be obtained, if it still exists, by calling it:
 138
 139 \begin{verbatim}
 140 >>> import weakref
 141 >>> class Object:
 142 ...     pass
 143 ...
 144 >>> o = Object()
 145 >>> r = weakref.ref(o)
 146 >>> o2 = r()
 147 >>> o is o2
 148 1
 149 \end{verbatim}
 150
 151 If the referent no longer exists, calling the reference object returns
 152 \code{None}:
 153
 154 \begin{verbatim}
 155 >>> del o, o2
 156 >>> print r()
 157 None
 158 \end{verbatim}
 159
 160 Testing that a weak reference object is still live should be done
 161 using the expression \code{\var{ref}() is not None}.  Normally,
 162 application code that needs to use a reference object should follow
 163 this pattern:
 164
 165 \begin{verbatim}
 166 # r is a weak reference object
 167 o = r()
 168 if o is None:
 169     # referent has been garbage collected
 170     print "Object has been allocated; can't frobnicate."
 171 else:
 172     print "Object is still live!"
 173     o.do_something_useful()
 174 \end{verbatim}
 175
 176 Using a separate test for ``liveness'' creates race conditions in
 177 threaded applications; another thread can cause a weak reference to
 178 become invalidated before the weak reference is called; the
 179 idiom shown above is safe in threaded applications as well as
 180 single-threaded applications.
 181
 182
 183 \subsection{Example \label{weakref-example}}
 184
 185 This simple example shows how an application can use objects IDs to
 186 retrieve objects that it has seen before.  The IDs of the objects can
 187 then be used in other data structures without forcing the objects to
 188 remain alive, but the objects can still be retrieved by ID if they
 189 do.
 190
 191 % Example contributed by Tim Peters <tim_one@msn.com>.
 192 \begin{verbatim}
 193 import weakref
 194
 195 _id2obj_dict = weakref.WeakValueDictionary()
 196
 197 def remember(obj):
 198     oid = id(obj)
 199     _id2obj_dict[oid] = obj
 200     return oid
 201
 202 def id2obj(oid):
 203     return _id2obj_dict[oid]
 204 \end{verbatim}
 205
 206
 207 \subsection{Weak References in Extension Types
 208             \label{weakref-extension}}
 209
 210 One of the goals of the implementation is to allow any type to
 211 participate in the weak reference mechanism without incurring the
 212 overhead on those objects which do not benefit by weak referencing
 213 (such as numbers).
 214
 215 For an object to be weakly referencable, the extension must include a
 216 \ctype{PyObject*} field in the instance structure for the use of the
 217 weak reference mechanism; it must be initialized to \NULL{} by the
 218 object's constructor.  It must also set the \member{tp_weaklistoffset}
 219 field of the corresponding type object to the offset of the field.
 220 For example, the instance type is defined with the following structure:
 221
 222 \begin{verbatim}
 223 typedef struct {
 224     PyObject_HEAD
 225     PyClassObject *in_class;       /* The class object */
 226     PyObject      *in_dict;        /* A dictionary */
 227     PyObject      *in_weakreflist; /* List of weak references */
 228 } PyInstanceObject;
 229 \end{verbatim}
 230
 231 The statically-declared type object for instances is defined this way:
 232
 233 \begin{verbatim}
 234 PyTypeObject PyInstance_Type = {
 235     PyObject_HEAD_INIT(&PyType_Type)
 236     0,
 237     "module.instance",
 238
 239     /* Lots of stuff omitted for brevity... */
 240
 241     offsetof(PyInstanceObject, in_weakreflist) /* tp_weaklistoffset */
 242 };
 243 \end{verbatim}
 244
 245 The only further addition is that the destructor needs to call the
 246 weak reference manager to clear any weak references.  This should be
 247 done before any other parts of the destruction have occurred, but is
 248 only required if the weak reference list is non-\NULL:
 249
 250 \begin{verbatim}
 251 static void
 252 instance_dealloc(PyInstanceObject *inst)
 253 {
 254     /* Allocate temporaries if needed, but do not begin
 255        destruction just yet.
 256      */
 257
 258     if (inst->in_weakreflist != NULL)
 259         PyObject_ClearWeakRefs((PyObject *) inst);
 260
 261     /* Proceed with object destruction normally. */
 262 }
 263 \end{verbatim}