[Python-Dev] PEP 246, redux

Alex Martelli aleax at aleax.it
Mon Jan 10 15:42:11 CET 2005

I had been promising to rewrite PEP 246 to incorporate the last several 
years' worth of discussions &c about it, and Guido's recent "stop the 
flames" artima blog post finally pushed me to complete the work.  
Feedback is of course welcome, so I thought I had better repost it 
here, rather than relying on would-be commenters to get it from CVS... 
I'm also specifically CC'ing Clark, the co-author, since he wasn't 
involved in this rewrite and of course I owe it to him to change or 
clearly attribute to myself anything he doesn't like to have "under his 
own name"!



PEP: 246
Title: Object Adaptation
Version: $Revision: 1.6 $
Author: aleax at aleax.it (Alex Martelli),
     cce at clarkevans.com (Clark C. Evans)
Status: Draft
Type: Standards Track
Created: 21-Mar-2001
Python-Version: 2.5
Post-History: 29-Mar-2001, 10-Jan-2005


     This proposal puts forth an extensible cooperative mechanism for
     the adaptation of an incoming object to a context which expects an
     object supporting a specific protocol (say a specific type, class,
     or interface).

     This proposal provides a built-in "adapt" function that, for any
     object X and any protocol Y, can be used to ask the Python
     environment for a version of X compliant with Y.  Behind the
     scenes, the mechanism asks object X: "Are you now, or do you know
     how to wrap yourself to provide, a supporter of protocol Y?".
     And, if this request fails, the function then asks protocol Y:
     "Does object X support you, or do you know how to wrap it to
     obtain such a supporter?"  This duality is important, because
     protocols can be developed after objects are, or vice-versa, and
     this PEP lets either case be supported non-invasively with regard
     to the pre-existing component[s].

     Lastly, if neither the object nor the protocol know about each
     other, the mechanism may check a registry of adapter factories,
     where callables able to adapt certain objects to certain protocols
     can be registered dynamically.  This part of the proposal is
     optional: the same effect could be obtained by ensuring that
     certain kinds of protocols and/or objects can accept dynamic
     registration of adapter factories, for example via suitable custom
     metaclasses.  However, this optional part allows adaptation to be
     made more flexible and powerful in a way that is not invasive to
     either protocols or other objects, thereby gaining for adaptation
     much the same kind of advantage that Python standard library's
     "copy_reg" module offers for serialization and persistence.

     This proposal does not specifically constrain what a protocol
     _is_, what "compliance to a protocol" exactly _means_, nor what
     precisely a wrapper is supposed to do.  These omissions are
     intended to leave this proposal compatible with both existing
     categories of protocols, such as the existing system of type and
     classes, as well as the many concepts for "interfaces" as such
     which have been proposed or implemented for Python, such as the
     one in PEP 245 [1], the one in Zope3 [2], or the ones discussed in
     the BDFL's Artima blog in late 2004 and early 2005 [3].  However,
     some reflections on these subjects, intended to be suggestive and
     not normative, are also included.


     Currently there is no standardized mechanism in Python for
     checking if an object supports a particular protocol.  Typically,
     existence of certain methods, particularly special methods such as
     __getitem__, is used as an indicator of support for a particular
     protocol.  This technique works well for a few specific protocols
     blessed by the BDFL (Benevolent Dictator for Life).  The same can
     be said for the alternative technique based on checking
     'isinstance' (the built-in class "basestring" exists specifically
     to let you use 'isinstance' to check if an object "is something
     like a string").  Neither approach is easily and generally
     extensible to other protocols, defined by applications and third
     party frameworks, outside of the standard Python core.

     Even more important than checking if an object already supports a
     given protocol can be the task of obtaining a suitable adapter
     (wrapper or proxy) for the object, if the support is not already
     there.  For example, a string does not support the file protocol,
     but you can wrap it into a StringIO instance to obtain an object
     which does support that protocol and gets its data from the string
     it wraps; that way, you can pass the string (suitably wrapped) to
     subsystems which require as their arguments objects that are
     readable as files.  Unfortunately, there is currently no general,
     standardized way to automate this extremely important kind of
     "adaptation by wrapping" operations.

     Typically, today, when you pass objects to a context expecting a
     particular protocol, either the object knows about the context and
     provides its own wrapper or the context knows about the object and
     wraps it appropriately.  The difficulty with these approaches is
     that such adaptations are one-offs, are not centralized in a
     single place of the users code, and are not executed with a common
     technique, etc.  This lack of standardization increases code
     duplication with the same adapter occurring in more than one place
     or it encourages classes to be re-written instead of adapted.  In
     either case, maintainability suffers.

     It would be very nice to have a standard function that can be
     called upon to verify an object's compliance with a particular
     protocol and provide for a wrapper if one is readily available --
     all without having to hunt through each library's documentation
     for the incantation appropriate to that particular, specific case.


     When considering an object's compliance with a protocol, there are
     several cases to be examined:

     a) When the protocol is a type or class, and the object has
        exactly that type or is an instance of exactly that class (not
        a subclass).  In this case, compliance is automatic.

     b) When the object knows about the protocol, and either considers
        itself compliant, or knows how to wrap itself suitably.

     c) When the protocol knows about the object, and either the object
        already complies or the protocol knows how to suitably wrap the

     d) When the protocol is a type or class, and the object is a
        member of a subclass.  This is distinct from the first case (a)
        above, since inheritance (unfortunately) does not necessarily
        imply substitutability, and thus must be handled carefully.

     e) When the context knows about the object and the protocol and
        knows how to adapt the object so that the required protocol is
        satisfied.  This could use an adapter registry or similar

     The fourth case above is subtle.  A break of substitutability can
     occur when a subclass changes a method's signature, or restricts
     the domains accepted for a method's argument ("co-variance" on
     arguments types), or extends the co-domain to include return
     values which the base class may never produce ("contra-variance"
     on return types).  While compliance based on class inheritance
     _should_ be automatic, this proposal allows an object to signal
     that it is not compliant with a base class protocol.

     If Python gains some standard "official" mechanism for interfaces,
     however, then the "fast-path" case (a) can and should be extended
     to the protocol being an interface, and the object an instance of
     a type or class claiming compliance with that interface.  For
     example, if the "interface" keyword discussed in [3] is adopted
     into Python, the "fast path" of case (a) could be used, since
     instantiable classes implementing an interface would not be
     allowed to break substitutability.


     This proposal introduces a new built-in function, adapt(), which
     is the basis for supporting these requirements.

     The adapt() function has three parameters:

     - `obj', the object to be adapted

     - `protocol', the protocol requested of the object

     - `alternate', an optional object to return if the object could
       not be adapted

     A successful result of the adapt() function returns either the
     object passed `obj', if the object is already compliant with the
     protocol, or a secondary object `wrapper', which provides a view
     of the object compliant with the protocol.  The definition of
     wrapper is deliberately vague, and a wrapper is allowed to be a
     full object with its own state if necessary.  However, the design
     intention is that an adaptation wrapper should hold a reference to
     the original object it wraps, plus (if needed) a minimum of extra
     state which it cannot delegate to the wrapper object.

     An excellent example of adaptation wrapper is an instance of
     StringIO which adapts an incoming string to be read as if it was a
     textfile: the wrapper holds a reference to the string, but deals
     by itself with the "current point of reading" (from _where_ in the
     wrapped strings will the characters for the next, e.g., "readline"
     call come from), because it cannot delegate it to the wrapped
     object (a string has no concept of "current point of reading" nor
     anything else even remotely related to that concept).

     A failure to adapt the object to the protocol raises an
     AdaptationError (which is a subclass of TypeError), unless the
     alternate parameter is used, in this case the alternate argument
     is returned instead.

     To enable the first case listed in the requirements, the adapt()
     function first checks to see if the object's type or the object's
     class are identical to the protocol.  If so, then the adapt()
     function returns the object directly without further ado.

     To enable the second case, when the object knows about the
     protocol, the object must have a __conform__() method.  This
     optional method takes two arguments:

     - `self', the object being adapted

     - `protocol, the protocol requested

     Just like any other special method in today's Python, __conform__
     is meant to be taken from the object's class, not from the object
     itself (for all objects, except instances of "classic classes" as
     long as we must still support the latter).  This enables a
     possible 'tp_conform' slot to be added to Python's type objects in
     the future, if desired.

     The object may return itself as the result of __conform__ to
     indicate compliance.  Alternatively, the object also has the
     option of returning a wrapper object compliant with the protocol.
     If the object knows it is not compliant although it belongs to a
     type which is a subclass of the protocol, then __conform__ should
     raise a LiskovViolation exception (a subclass of AdaptationError).
     Finally, if the object cannot determine its compliance, it should
     return None to enable the remaining mechanisms.  If __conform__
     raises any other exception, "adapt" just propagates it.

     To enable the third case, when the protocol knows about the
     object, the protocol must have an __adapt__() method.  This
     optional method takes two arguments:

     - `self', the protocol requested

     - `obj', the object being adapted

     If the protocol finds the object to be compliant, it can return
     obj directly.  Alternatively, the method may return a wrapper
     compliant with the protocol.  If the protocol knows the object is
     not compliant although it belongs to a type which is a subclass of
     the protocol, then __adapt__ should raise a LiskovViolation
     exception (a subclass of AdaptationError).  Finally, when
     compliance cannot be determined, this method should return None to
     enable the remaining mechanisms.  If __adapt__ raises any other
     exception, "adapt" just propagates it.

     The fourth case, when the object's class is a sub-class of the
     protocol, is handled by the built-in adapt() function.  Under
     normal circumstances, if "isinstance(object, protocol)" then
     adapt() returns the object directly.  However, if the object is
     not substitutable, either the __conform__() or __adapt__()
     methods, as above mentioned, may raise an LiskovViolation (a
     subclass of AdaptationError) to prevent this default behavior.

     If none of the first four mechanisms worked, as a last-ditch
     attempt, 'adapt' falls back to checking a registry of adapter
     factories, indexed by the protocol and the type of `obj', to meet
     the fifth case.  Adapter factories may be dynamically registered
     and removed from that registry to provide "third party adaptation"
     of objects and protocols that have no knowledge of each other, in
     a way that is not invasive to either the object or the protocols.

Intended Use

     The typical intended use of adapt is in code which has received
     some object X "from the outside", either as an argument or as the
     result of calling some function, and needs to use that object
     according to a certain protocol Y.  A "protocol" such as Y is
     meant to indicate an interface, usually enriched with some
     semantics constraints (such as are typically used in the "design
     by contract" approach), and often also some pragmatical
     expectation (such as "the running time of a certain operation
     should be no worse than O(N)", or the like); this proposal does
     not specify how protocols are designed as such, nor how or whether
     compliance to a protocol is checked, nor what the consequences may
     be of claiming compliance but not actually delivering it (lack of
     "syntactic" compliance -- names and signatures of methods -- will
     often lead to exceptions being raised; lack of "semantic"
     compliance may lead to subtle and perhaps occasional errors
     [imagine a method claiming to be threadsafe but being in fact
     subject to some subtle race condition, for example]; lack of
     "pragmatic" compliance will generally lead to code that runs
     ``correctly'', but too slowly for practical use, or sometimes to
     exhaustion of resources such as memory or disk space).

     When protocol Y is a concrete type or class, compliance to it is
     intended to mean that an object allows all of the operations that
     could be performed on instances of Y, with "comparable" semantics
     and pragmatics.  For example, a hypothetical object X that is a
     singly-linked list should not claim compliance with protocol
     'list', even if it implements all of list's methods: the fact that
     indexing X[n] takes time O(n), while the same operation would be
     O(1) on a list, makes a difference.  On the other hand, an
     instance of StringIO.StringIO does comply with protocol 'file',
     even though some operations (such as those of module 'marshal')
     may not allow substituting one for the other because they perform
     explicit type-checks: such type-checks are "beyond the pale" from
     the point of view of protocol compliance.

     While this convention makes it feasible to use a concrete type or
     class as a protocol for purposes of this proposal, such use will
     often not be optimal.  Rarely will the code calling 'adapt' need
     ALL of the features of a certain concrete type, particularly for
     such rich types as file, list, dict; rarely can all those features
     be provided by a wrapper with good pragmatics, as well as syntax
     and semantics that are really the same as a concrete type's.

     Rather, once this proposal is accepted, a design effort needs to
     start to identify the essential characteristics of those protocols
     which are currently used in Python, particularly within the
     standard library, and to formalize them using some kind of
     "interface" construct (not necessarily requiring any new syntax: a
     simple custom metaclass would let us get started, and the results
     of the effort could later be migrated to whatever "interface"
     construct is eventually accepted into the Python language).  With
     such a palette of more formally designed protocols, the code using
     'adapt' will be able to ask for, say, adaptation into "a filelike
     object that is readable and seekable", or whatever else it
     specifically needs with some decent level of "granularity", rather
     than too-generically asking for compliance to the 'file' protocol.

     Adaptation is NOT "casting".  When object X itself does not
     conform to protocol Y, adapting X to Y means using some kind of
     wrapper object Z, which holds a reference to X, and implements
     whatever operation Y requires, mostly by delegating to X in
     appropriate ways.  For example, if X is a string and Y is 'file',
     the proper way to adapt X to Y is to make a StringIO(X), *NOT* to
     call file(X) [which would try to open a file named by X].

     Numeric types and protocols may need to be an exception to this
     "adaptation is not casting" mantra, however.

Guido's "Optional Static Typing: Stop the Flames" Blog Entry

     A typical simple use case of adaptation would be:

         def f(X):
             X = adapt(X, Y)
             # continue by using X according to protocol X

     In [4], the BDFL has proposed introducing the syntax:

         def f(X: Y):
             # continue by using X according to protocol X

     to be a handy shortcut for exactly this typical use of adapt, and,
     as a basis for experimentation until the parser has been modified
     to accept this new syntax, a semantically equivalent decorator:

         def f(X):
             # continue by using X according to protocol X

     These BDFL ideas are fully compatible with this proposal, as are
     other of Guido's suggestions in the same blog.

Reference Implementation and Test Cases

     The following reference implementation does not deal with classic
     classes: it consider only new-style classes.  If classic classes
     need to be supported, the additions should be pretty clear, though
     a bit messy (x.__class__ vs type(x), getting boundmethods directly
     from the object rather than from the type, and so on).

     class AdaptationError(TypeError):
     class LiskovViolation(AdaptationError):

     _adapter_factory_registry = {}

     def registerAdapterFactory(objtype, protocol, factory):
         _adapter_factory_registry[objtype, protocol] = factory

     def unregisterAdapterFactory(objtype, protocol):
         del _adapter_factory_registry[objtype, protocol]

     def _adapt_by_registry(obj, protocol, alternate):
         factory = _adapter_factory_registry.get((type(obj), protocol))
         if factory is None:
             adapter = alternate
             adapter = factory(obj, protocol, alternate)
         if adapter is AdaptationError:
             raise AdaptationError
             return adapter

     def adapt(obj, protocol, alternate=AdaptationError):

         t = type(obj)

         # (a) first check to see if object has the exact protocol
         if t is protocol:
            return obj

             # (b) next check if t.__conform__ exists & likes protocol
             conform = getattr(t, '__conform__', None)
             if conform is not None:
                 result = conform(obj, protocol)
                 if result is not None:
                     return result

             # (c) then check if protocol.__adapt__ exists & likes obj
             adapt = getattr(type(protocol), '__adapt__', None)
             if adapt is not None:
                 result = adapt(protocol, obj)
                 if result is not None:
                     return result
         except LiskovViolation:
             # (d) check if object is instance of protocol
             if isinstance(obj, protocol):
                 return obj

         # (e) last chance: try the registry
         return _adapt_by_registry(obj, protocol, alternate)

     from adapt import AdaptationError, LiskovViolation, adapt
     from adapt import registerAdapterFactory, unregisterAdapterFactory
     import doctest

     class A(object):
         >>> a = A()
         >>> a is adapt(a, A)   # case (a)

     class B(A):
         >>> b = B()
         >>> b is adapt(b, A)   # case (d)

     class C(object):
         >>> c = C()
         >>> c is adapt(c, B)   # case (b)
         >>> c is adapt(c, A)   # a failure case
         Traceback (most recent call last):
         def __conform__(self, protocol):
             if protocol is B:
                 return self

     class D(C):
         >>> d = D()
         >>> d is adapt(d, D)   # case (a)
         >>> d is adapt(d, C)   # case (d) explicitly blocked
         Traceback (most recent call last):
         def __conform__(self, protocol):
             if protocol is C:
                 raise LiskovViolation

     class MetaAdaptingProtocol(type):
         def __adapt__(cls, obj):
             return cls.adapt(obj)

     class AdaptingProtocol:
         __metaclass__ = MetaAdaptingProtocol
         def adapt(cls, obj):

     class E(AdaptingProtocol):
         >>> a = A()
         >>> a is adapt(a, E)   # case (c)
         >>> b = A()
         >>> b is adapt(b, E)   # case (c)
         >>> c = C()
         >>> c is adapt(c, E)   # a failure case
         Traceback (most recent call last):
         def adapt(cls, obj):
             if isinstance(obj, A):
                 return obj

     class F(object):

     def adapt_F_to_A(obj, protocol, alternate):
         if isinstance(obj, F) and issubclass(protocol, A):
             return obj
             return alternate

     def test_registry():
         >>> f = F()
         >>> f is adapt(f, A)   # a failure case
         Traceback (most recent call last):
         >>> registerAdapterFactory(F, A, adapt_F_to_A)
         >>> f is adapt(f, A)   # case (e)
         >>> unregisterAdapterFactory(F, A)
         >>> f is adapt(f, A)   # a failure case again
         Traceback (most recent call last):
         >>> registerAdapterFactory(F, A, adapt_F_to_A)


Relationship To Microsoft's QueryInterface

     Although this proposal has some similarities to Microsoft's (COM)
     QueryInterface, it differs by a number of aspects.

     First, adaptation in this proposal is bi-directional, allowing the
     interface (protocol) to be queried as well, which gives more
     dynamic abilities (more Pythonic).  Second, there is no special
     "IUnknown" interface which can be used to check or obtain the
     original unwrapped object identity, although this could be
     proposed as one of those "special" blessed interface protocol
     identifiers.  Third, with QueryInterface, once an object supports
     a particular interface it must always there after support this
     interface; this proposal makes no such guarantee, since, in
     particular, adapter factories can be dynamically added to the
     registried and removed again later.

     Fourth, implementations of Microsoft's QueryInterface must support
     a kind of equivalence relation -- they must be reflexive,
     symmetrical, and transitive, in specific senses.  The equivalent
     conditions for protocol adaptation according to this proposal
     would also represent desirable properties:

         # given, to start with, a successful adaptation:
         X_as_Y = adapt(X, Y)

         # reflexive:
         assert adapt(X_as_Y, Y) is X_as_Y

         # transitive:
         X_as_Z = adapt(X, Z, None)
         X_as_Y_as_Z = adapt(X_as_Y, Z, None)
         assert (X_as_Y_as_Z is None) == (X_as_Z is None)

         # symmetrical:
         X_as_Z_as_Y = adapt(X_as_Z, Y, None)
         assert (X_as_Y_as_Z is None) == (X_as_Z_as_Y is None)

     However, while these properties are desirable, it may not be
     possible to guarantee them in all cases.  QueryInterface can
     impose their equivalents because it dictates, to some extent, how
     objects, interfaces, and adapters are to be coded; this proposal
     is meant to be not necessarily invasive, usable and to "retrofit"
     adaptation between two frameworks coded in mutual ignorance of
     each other without having to modify either framework.

     Transitivity of adaptation is in fact somewhat controversial, as
     is the relationship (if any) between adaptation and inheritance.

     The latter would not be controversial if we knew that inheritance
     always implies Liskov substitutability, which, unfortunately we
     don't.  If some special form, such as the interfaces proposed in
     [4], could indeed ensure Liskov substitutability, then for that
     kind of inheritance, only, we could perhaps assert that if X
     conforms to Y and Y inherits from Z then X conforms to Z... but
     only if substitutability was taken in a very strong sense to
     include semantics and pragmatics, which seems doubtful.  (For what
     it's worth: in QueryInterface, inheritance does not require nor
     imply conformance).  This proposal does not include any "strong"
     effects of inheritance, beyond the small ones specifically
     detailed above.

     Similarly, transitivity might imply multiple "internal" adaptation
     passes to get the result of adapt(X, Z) via some intermediate Y,
     intrinsically like adapt(adapt(X, Y), Z), for some suitable and
     automatically chosen Y.  Again, this may perhaps be feasible under
     suitably strong constraints, but the practical implications of
     such a scheme are still unclear to this proposal's authors.  Thus,
     this proposal does not include any automatic or implicit
     transitivity of adaptation, under whatever circumstances.

     For an implementation of the original version of this proposal
     which performs more advanced processing in terms of transitivity,
     and of the effects of inheritance, see Phillip J. Eby's
     PyProtocols [5].  The documentation accompanying PyProtocols is
     well worth studying for its considerations on how adapters should
     be coded and used, and on how adaptation can remove any need for
     typechecking in application code.

Questions and Answers

     Q:  What benefit does this proposal provide?

     A:  The typical Python programmer is an integrator, someone who is
         connecting components from various suppliers.  Often, to
         interface between these components, one needs intermediate
         adapters.  Usually the burden falls upon the programmer to
         study the interface exposed by one component and required by
         another, determine if they are directly compatible, or develop
         an adapter.  Sometimes a supplier may even include the
         appropriate adapter, but even then searching for the adapter
         and figuring out how to deploy the adapter takes time.

         This technique enables supplierrs to work with each other
         directly, by implementing __conform__ or __adapt__ as
         necessary.  This frees the integrator from making their own
         adapters.  In essence, this allows the components to have a
         simple dialogue among themselves.  The integrator simply
         connects one component to another, and if the types don't
         automatically match an adapting mechanism is built-in.

         Moreover, thanks to the adapter registry, a "fourth party" may
         supply adapters to allow interoperation of frameworks which
         are totally unaware of each other, non-invasively, and without
         requiring the integrator to do anything more than install the
         appropriate adapter factories in the registry at start-up.

         As long as libraries and frameworks cooperate with the
         adaptation infrastructure proposed here (essentially by
         defining and using protocols appropriately, and calling
         'adapt' as needed on arguments received and results of
         call-back factory functions), the integrator's work thereby
         becomes much simpler.

         For example, consider SAX1 and SAX2 interfaces: there is an
         adapter required to switch between them.  Normally, the
         programmer must be aware of this; however, with this
         adaptation proposal in place, this is no longer the case --
         indeed, thanks to the adapter registry, this need may be
         removed even if the framework supplying SAX1 and the one
         requiring SAX2 are unaware of each other.

     Q:  Why does this have to be built-in, can't it be standalone?

     A:  Yes, it does work standalone.  However, if it is built-in, it
         has a greater chance of usage.  The value of this proposal is
         primarily in standardization: having libraries and frameworks
         coming from different suppliers, including the Python standard
         library, use a single approach to adaptation.  Furthermore:

         0.  The mechanism is by its very nature a singleton.

         1.  If used frequently, it will be much faster as a built-in.

         2.  It is extensible and unassuming.

         3.  Once 'adapt' is built-in, it can support syntax extensions
             and even be of some help to a type inference system.

     Q:  Why the verbs __conform__ and __adapt__?

     A:  conform, verb intransitive
             1. To correspond in form or character; be similar.
             2. To act or be in accord or agreement; comply.
             3. To act in accordance with current customs or modes.

         adapt, verb transitive
             1. To make suitable to or fit for a specific use or

         Source:  The American Heritage Dictionary of the English
                  Language, Third Edition

Backwards Compatibility

     There should be no problem with backwards compatibility unless
     someone had used the special names __conform__ or __adapt__ in
     other ways, but this seems unlikely, and, in any case, user code
     should never use special names for non-standard purposes.

     This proposal could be implemented and tested without changes to
     the interpreter.


     This proposal was created in large part by the feedback of the
     talented individuals on the main Python mailing lists and the
     type-sig list.  To name specific contributors (with apologies if
     we missed anyone!), besides the proposal's authors: the main
     suggestions for the proposal's first versions came from Paul
     Prescod, with significant feedback from Robin Thomas, and we also
     borrowed ideas from Marcin 'Qrczak' Kowalczyk and Carlos Ribeiro.

     Other contributors (via comments) include Michel Pelletier, Jeremy
     Hylton, Aahz Maruch, Fredrik Lundh, Rainer Deyke, Timothy Delaney,
     and Huaiyu Zhu.  The current version owes a lot to discussions
     with (among others) Phillip J. Eby, Guido van Rossum, Bruce Eckel,
     Jim Fulton, and Ka-Ping Yee, and to study and reflection of their
     proposals, implementations, and documentation about use and
     adaptation of interfaces and protocols in Python.

References and Footnotes

     [1] PEP 245, Python Interface Syntax, Pelletier

     [2] http://www.zope.org/Wikis/Interfaces/FrontPage

     [3] http://www.artima.com/weblogs/index.jsp?blogger=guido

     [4] http://www.artima.com/weblogs/viewpost.jsp?thread=87182

     [5] http://peak.telecommunity.com/PyProtocols.html


     This document has been placed in the public domain.

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