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
+In the following, the term \dfn{referent} means the
object which is referred to by a weak reference.
-XXX --- need to say more here!
+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 \class{WeakKeyDictionary} and
+\class{WeakValueDictionary} classes supplied by the \module{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. 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 \module{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),
error output, but cannot be propagated; they are handled in exactly
the same way as exceptions raised from an object's
\method{__del__()} method.
-
+
Weak references are hashable if the \var{object} is hashable. They
will maintain their hash value even after the \var{object} was
deleted. If \function{hash()} is called the first time only after
the \var{object} was deleted, the call will raise
\exception{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
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).}
\end{classdesc}
\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 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).}
\end{classdesc}
\begin{datadesc}{ReferenceType}
list to \NULL{}:
\begin{verbatim}
-static PyObject *
-instance_new() {
- /* Other initialization stuff omitted for brevity */
+static PyObject *
+instance_new() {
+ /* Other initialization stuff omitted for brevity */
- self->in_weakreflist = NULL;
+ self->in_weakreflist = NULL;
- return (PyObject *) self;
-}
+ return (PyObject *) self;
+}
\end{verbatim}
The only further addition is that the destructor needs to call the
projects / thirdparty / Python / cpython.git / commitdiff
