155 lines
6.0 KiB
Plaintext
155 lines
6.0 KiB
Plaintext
PEP: 205
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Title: Weak References
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Version: $Revision$
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Owner: Fred L. Drake, Jr. <fdrake@acm.org>
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Python-Version: 2.1
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Status: Incomplete
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Type: Standards Track
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Post-History:
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Motivation
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There are two basic applications for weak references which have
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been noted by Python programmers: object caches and reduction of
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pain from circular references.
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Caches (weak dictionaries)
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There is a need to allow objects to be maintained to represent
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external state, mapping a single instance to the external
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reality, where allowing multiple instances to be mapped to the
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same external resource would create unnecessary difficulty
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maintaining synchronization among instances. In these cases,
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a common idiom is to support a cache of instances; a factory
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function is used to return either a new or existing instance.
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The difficulty in this approach is that one of two things must
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be tolerated: either the cache grows without bound, or there
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needs to be explicit management of the cache elsewhere in the
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application. The later can be very tedious and leads to more
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code than is really necessary to solve the problem at hand,
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and the former can be unacceptable for long-running processes
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or even relatively short processes with substantial memory
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requirements.
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- External objects that need to be represented by a single
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instance, no matter how many internal users there are. This
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can be useful for representing files that need to be written
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back to disk in whole rather than locked & modified for
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every use.
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- Objects which are expensive to create, but may be needed by
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multiple internal consumers. Similar to the first case, but
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not necessarily bound to external resources, and possibly
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not an issue for shared state. Weak references are only
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useful in this case if there is some flavor of "soft"
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references or if there is a high likelihood that users of
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individual objects will overlap in lifespan.
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Circular references
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- DOMs require a huge amount of circular (to parent & document
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nodes), but most of these aren't useful. Using weak
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references allows applications to hold onto less of the tree
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without a lot of difficulty. This might be especially
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useful in the context of something like xml.dom.pulldom.
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Weak References in Java
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http://java.sun.com/j2se/1.3/docs/api/java/lang/ref/package-summary.html
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Java provides three forms of weak references, and one interesting
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helper class. The three forms are called "weak", "soft", and
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"phantom" references. The relevant classes are defined in the
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java.lang.ref package.
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For each of the reference types, there is an option to add the
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reference to a queue when it is invalidated by the memory
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allocator. The primary purpose of this facility seems to be that
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it allows larger structures to be composed to incorporate
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weak-reference semantics without having to impose substantial
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additional locking requirements. For instance, it would not be
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difficult to use this facility to create a "weak" hash table which
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removes keys and referents when a reference is no longer used
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elsewhere. Using weak references for the objects without some
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sort of notification queue for invalidations leads to much more
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tedious implementation of the various operations required on hash
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tables. This can be a performance bottleneck if deallocations of
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the stored objects are infrequent.
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Java's "weak" references are most like Diane Hackborn's old vref
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proposal: a reference object refers to a single Python object,
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but does not own a reference to that object. When that object is
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deallocated, the reference object is invalidated. Users of the
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reference object can easily determine that the reference has been
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invalidated, or a NullObjectDereferenceError can be raised when
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an attempt is made to use the referred-to object.
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The "soft" references are similar, but are not invalidated as soon
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as all other references to the referred-to object have been
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released. The "soft" reference does own a reference, but allows
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the memory allocator to free the referent if the memory is needed
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elsewhere. It is not clear whether this means soft references are
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released before the malloc() implementation calls sbrk() or its
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equivalent, or if soft references are only cleared when malloc()
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returns NULL.
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XXX -- Need to figure out what phantom references are all about.
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Unlike the other two reference types, "phantom" references must be
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associated with an invalidation queue.
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Previous Weak Reference Work in Python
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Diane Hackborn's vref proposal. 'vref' objects were very similar
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to java.lang.ref.WeakReference objects, except there was no
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equivalent to the invalidation queues. Implementing a "weak
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dictionary" would be just as difficult as using only weak
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references (without the invalidation queue) in Java. Information
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on this has disappeared from the Web. Original discussion
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occurred in the comp.lang.python newsgroup; a good archive of that
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may turn up something more.
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Dianne doesn't have any record of her proposal, and doesn't recall
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doing an implementation.
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Marc-Andr<64> Lemburg's mx.Proxy package. These Web pages appear to
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be unavailable at the moment.
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http://starship.python.net/crew/lemburg/
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The weakdict module by Dieter Maurer is implemented in C and
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Python. It appears that the Web pages have not been updated since
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Python 1.5.2a, so I'm not yet sure if the implementation is
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compatible with Python 2.0.
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http://www.handshake.de/~dieter/weakdict.html
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Possible Applications
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PyGTK+ bindings?
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Tkinter?
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DOM trees?
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Proposed Implementation
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XXX -- Not yet.
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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