diff --git a/Doc/lib/libweakref.tex b/Doc/lib/libweakref.tex index 73caf94bea5..7f851cdef33 100644 --- a/Doc/lib/libweakref.tex +++ b/Doc/lib/libweakref.tex @@ -14,6 +14,9 @@ The \module{weakref} module allows the Python programmer to create \dfn{weak references} to objects. +In the discussion which follows, the term \dfn{referent} means the +object which is referred to by a weak reference. + XXX --- need to say more here! Not all objects can be weakly referenced; those objects which do @@ -24,12 +27,13 @@ be made to support weak references; see section \ref{weakref-extension}, \begin{funcdesc}{ref}{object\optional{, callback}} - Return a weak reference to \var{object}. If \var{callback} is + Return a weak reference to \var{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 \code{None} to be returned. If \var{callback} is provided, it 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. - The original object can be retrieved by calling the reference - object, if the referent is still alive. It is allowable for many weak references to be constructed for the same object. Callbacks registered for each weak reference will be @@ -48,10 +52,11 @@ be made to support weak references; see section \ref{weakref-extension}, \exception{TypeError}. Weak references support tests for equality, but not ordering. If - the \var{object} is still alive, two references are equal if the - objects are equal (regardless of the \var{callback}). If - \var{object} has been deleted, they are equal only if the references - being compared are the same reference object. + the referents are still alive, two references have the same + equalality relationship as their referents (regardless of the + \var{callback}). If either referent has been deleted, the + references are equal only if the reference objects are the same + object. \end{funcdesc} \begin{funcdesc}{proxy}{object\optional{, callback}} @@ -89,7 +94,7 @@ be made to support weak references; see section \ref{weakref-extension}, \begin{classdesc}{WeakValueDictionary}{\optional{dict}} Mapping class that references values weakly. Entries in the dictionary will be discarded when no strong reference to the value - exists anymore. + exists any more. \end{classdesc} \begin{datadesc}{ReferenceType} @@ -158,7 +163,8 @@ application code that needs to use a reference object should follow this pattern: \begin{verbatim} -o = ref() +# r is a weak reference object +o = r() if o is None: # referent has been garbage collected print "Object has been allocated; can't frobnicate." @@ -169,7 +175,7 @@ else: Using a separate test for ``liveness'' creates race conditions in threaded applications; another thread can cause a weak reference to -become invalidated before the \method{get()} method is called; the +become invalidated before the weak reference is called; the idiom shown above is safe in threaded applications as well as single-threaded applications. @@ -189,10 +195,12 @@ import weakref _id2obj_dict = weakref.WeakValueDictionary() def remember(obj): - _id2obj_dict[id(obj)] = obj + oid = id(obj) + _id2obj_dict[oid] = obj + return oid -def id2obj(id): - return _id2obj_dict(id) +def id2obj(oid): + return _id2obj_dict[oid] \end{verbatim} @@ -205,7 +213,7 @@ overhead on those objects which do not benefit by weak referencing (such as numbers). For an object to be weakly referencable, the extension must include a -\ctype{PyObject *} field in the instance structure for the use of the +\ctype{PyObject*} field in the instance structure for the use of the weak reference mechanism; it must be initialized to \NULL{} by the object's constructor. It must also set the \member{tp_weaklistoffset} field of the corresponding type object to the offset of the field. @@ -236,17 +244,19 @@ PyTypeObject PyInstance_Type = { The only further addition is that the destructor needs to call the weak reference manager to clear any weak references. This should be -done before any other parts of the destruction have occurred: +done before any other parts of the destruction have occurred, but is +only required if the weak reference list is non-\NULL: \begin{verbatim} static void instance_dealloc(PyInstanceObject *inst) { - /* Allocate tempories if needed, but do not begin + /* Allocate temporaries if needed, but do not begin destruction just yet. */ - PyObject_ClearWeakRefs((PyObject *) inst); + if (inst->in_weakreflist != NULL) + PyObject_ClearWeakRefs((PyObject *) inst); /* Proceed with object destruction normally. */ }