Yet Another PEP: Query Protocol Interface or __query__
Clark C. Evans
cce at clarkevans.com
Sat Mar 24 22:22:20 EST 2001
Below is an updated version of the proposal Ive been
a champion for. It incorporates a great deal of feedback
that Ive received. Changes include:
* By default, adapt now throws an error again, however
an alternate object can be provided upon failure much
like getattr.
* Now __adapt__ can either return None or throw a TypeError
with the same effect.
* This now includes reverse adaptation of the object
by the protocol via __prodapt__ (better name?), which
is very similar to Paul Prescods __check__
* The text has been tightened (hopefully eliminating
much superfluous material)
* The sample implementation code has been cleaned up
and tested in more detail.
* Ive received much comment to remove the type-checking
stuff. I have a question: Is type-checking a component
of adapting?
I would very much appreciate any comments and feedback.
Especially if the intent/value of this proposal is not
clear as I seem to be having a difficult time with the
expression of this.
Further, I still do not have a PEP number. Either I have
not asked correctly, am being stupid, have pissed someone
off, or am proposing something that has already been
rejected. In any case, what do I do?
Kind regards,
Clark
P.S. Id like to thank Alex Martelli for his most helpful
comments and hope that I have dutifully incorporated
his suggestions appropriately.
PEP: XXX
Title: Protocol Checking and Adaptation
Version: $Revision$
Author: Clark Evans
Python-Version: 2.2
Status: Draft
Type: Standards Track
Created: 21-Mar-2001
Updated: 23-Mar-2001
Abstract
This proposal puts forth a light-weight explicit mechanism
for the adaptation (including verification) of an object to
a context where a specific type, class, interface, or other
protocol is expected. This proposal can leverage existing
protocols such as the type system and class hierarchy and is
orthogonal, if not complementary to the pending interface
mechanism [1] and signature based type-checking system [2]
This proposal allows a standard and extensible method to
ask two types of questions: Is a particular object
compliant with a given protocol? (b) And if not, can the
object be wrapped so that it is complaint? This proposal
does not limit what a protocol is, what compliance to the
protocol means, nor what a wrapper constitutes.
Motivation
Currently there is no standardized mechanism in Python for
asking if an object supports a particular protocol. Typically,
existence of particular methods, particularly those that are
built-in such as __getitem__, is used as an indicator of
support for a particular protocol. This technique works for
protocols blessed by GvR, such as the new enumerator proposal
identified by a new built-in __iter__. However, this technique
does not admit an infallible way to identify interfaces lacking
a unique, built-in signature method.
More so, there is no standardized way to obtain an adapter
for an object. Typically, with objects passed 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 objects compliance with a
particular protocol and provide for a wrapper if one is
readily available -- all without having to hunt through
a librarys documentation for the appropriate incantation.
Requirements
When considering an objects 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 a member of the class.
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
appropriately.
c) When the protocol knows about the object and either
it complies or can be wrapped accordingly.
d) When the protocol is a class, and the object is a
member of a subclass. This is distinct from the first
case (a) above, since inheritance does not necessarily
imply substitutability and 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 method.
For this proposal's requirements, the first case should be
come for free and the next three cases should be relatively
relatively easy to accomplish. This proposal does not address
the last case, however it provides a base mechanism upon which
such an approach could be developed. Further, with only minor
implementation changes, this proposal should be able to
incorporate a new interface type or type checking system.
The fourth case above is subtle. A lack of substitutability can
occur when a method restricts an argument's domain or raises an
exception which a base class does not or extends the co-domain to
include return values which the base class may never produce.
While compliance based on class inheritance should be automatic,
this proposal should allow an object to signal that it is not
compliant with a base class protocol.
Specification
This proposal introduces a new built-in function, adapt, which
is the basis for supporting these requirements.
The adapt function has four parameters: 1) the object to be
adapted ("obj"), 2) the protocol requested of the object
("protocol"), 3) an optional object to return if the object
could not be adapted ("alternate"), and 4) an optional flag
which can be used to prevent a wrapper from being generated
("can_wrap") if provided.
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 explicitly vague and a wrapper
is allowed to be a full object with its own state if necessary.
A failure to adapt the object to the protocol will raise a
TypeError unless the alternate parameter is used, in this
case the alternate argument is returned.
To enable the first case listed in the requirements,
the adapt function first checks to see if the objects
type or the objects 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 an __adapt__ method. This
optional method takes three arguments, the object being adapted
("self"), the protocol requested ("protocol"), and a flag
("can_wrap"). The object may return itself through this method
to indicate compliance. Alternatively, if the can_wrap flag is
true the object also has the option of returning a wrapper object
compliant with the protocol. Finally, if the object cannot
determine its compliance, it should either return None or
raise a TypeError to allow other mechanisms to be checked.
To enable the third case, when the protocol knows about the object,
the protocol must have a __prodapt__ method. This optional method
takes three arguments, the protocol requested ("self"), the
object being adapted ("obj"), and a flag ("can_wrap"). If the
protocol finds the object to be compliant, it can return obj
directly. Alternatively, if the can_wrap flag is true, the
method may return a wrapper compliant with the protocol. Finally,
compliance cannot be determined, this method should either
return None or raise a TypeError so other mechanisms can be tried.
The fourth case, when the objects class is a sub-class of the
protocol, is handled by the built-in adapt function. Under normal
circumstances, if isinstance(object,protcol) then adapt returns
the object directly. However, if the object is not substitutable,
either the __adapt__ or __prodapt__ methods above may raise an
adaptForceFailException to prevent this default behavior.
This proposal could introduce one more built-in function, "isa",
perhaps implemented as an operator. If this is deemed useful, the
function would take two arguments, the first is the object to be
checked ("obj"), and the second is the protocol to check the
object against ("protocol"). The return value of the operator
will be either be the obj or None. This function would be
implemented by calling the adapt function with the can_wrap being
false and an alternate argument of None.
Please note two important things. First, this proposal does not
preclude the addition of other protocols. Second, this proposal
does not preclude other possible cases where adapter pattern may
hold, such as the context knowing the object and the protocol
(the last case in the requirements). In fact, this proposal opens
the gate for these other mechanisms to be added; while keeping the
change in manageable chunks.
Reference Implementation and Example Usage
-----------------------------------------------------------------
adapter.py
-----------------------------------------------------------------
import types
adaptRaiseTypeException = "(raise a type exception on failure)"
adaptForceFailException = "(forced failure of adapt)"
def adapt(obj, protocol, alternate = \
adaptRaiseTypeException, can_wrap = 1):
# first check to see if object has the exact protocol
if type(obj) is types.InstanceType and \
obj.__class__ is protocol: return obj
if type(obj) is protocol: return obj
# procedure to execute on success
def succeed(retval,obj,protocol,can_wrap):
if can_wrap: return retval
return obj
# procedure to execute on failure
def fail(obj,protocol,alternate):
if alternate is adaptRaiseTypeException:
raise TypeError("%s cannot be adapted to %s" \
% (obj,protocol))
return alternate
# try to use the object's adapting mechanism
adapt = getattr(obj, '__adapt__',None)
if adapt:
try:
retval = adapt(protocol,can_wrap)
if retval: return succeed(retval,obj,protocol,can_wrap)
except adaptForceFailException:
return fail(obj,protocol,alternate)
except TypeError: pass
# try to use the protocol's adapting mechanism
adapt = getattr(protocol, '__prodapt__',None)
if adapt:
try:
retval = adapt(obj,can_wrap)
if retval: return succeed(retval,obj,protocol,can_wrap)
except adaptForceFailException:
return fail(obj,protocol,alternate)
except TypeError: pass
# check to see if the object is an instance
try:
if isinstance(obj,protocol):
return obj
except TypeError: pass
# no-adaptation-possible case
return fail(obj,protocol,alternate)
# imagine binary operator syntax
def isa(obj,protocol):
return adapt(obj,protocol,None,0)
-----------------------------------------------------------------
test.py
-----------------------------------------------------------------
import adapter
import types
from adapter import adapt
from adapter import isa
from adapter import adaptForceFailException
class KnightsWhoSayNi: pass
class Eggs: # an unrelated class/interface
def eggs(self): print "eggs!"
word = "Nee-womm"
class Ham: # used as an interface, no inhertance
def ham(self): pass
word = "Ping"
class Spam: # a base class, inheritance used
def spam(self): print "spam!"
class EggsSpamAndHam (Spam,KnightsWhoSayNi):
def ham(self): print "ham!"
def __adapt__(self,protocol,can_wrap):
if protocol is Ham:
# implements Ham's ham, but does not have a word
return self
if protocol is KnightsWhoSayNi:
# we are no longer the Knights who say Ni!
raise adaptForceFailException
if protocol is Eggs and can_wrap:
# Knows how to create the eggs!
return Eggs()
raise TypeError("haha")
class Bing (Ham):
def __adapt__(self,protcol,can_wrap):
raise adaptForceFailException
# in this case, it is the protcol which determines
# if the class is adapted
class SacredWord:
class HasSecredWord:
def __call__(self, obj, can_wrap):
if getattr(obj,'word',None): return obj
__prodapt__= HasSecredWord()
def test():
x = EggsSpamAndHam()
adapt(x,Spam).spam()
adapt(x,Eggs).eggs()
adapt(x,Ham).ham()
adapt(x,EggsSpamAndHam).ham()
print adapt(Eggs(),SacredWord).word
print adapt(Ham(),SacredWord).word
pass
if adapt(x,KnightsWhoSayNi,None): raise "IckyIcky"
if not isa(x,Spam): raise "Spam"
if isa(x,Eggs): raise "Eggs"
if not isa(x,Ham): raise "Ham"
if not isa(x,EggsSpamAndHam): raise "EggsAndSpam"
if isa(x,KnightsWhoSayNi): raise "NightsWhoSayNi"
if isa(x,SacredWord): raise "SacredWord"
try:
adapt(x,SacredWord)
except TypeError: pass
else: raise "SacredWord"
try:
adapt(x,KnightsWhoSayNi)
except TypeError: print "Ekky-ekky-ekky-ekky-z'Bang, " \
+ "zoom-Boing, z'nourrrwringmm"
else: raise "NightsWhoSayNi"
pass
b = Bing()
if not isa(b,Bing): raise "Not a Bing"
if isa(b,Ham): raise "Not a Ham!"
if isa(1,types.FloatType): raise "Not a float!"
if isa(b,types.FloatType): raise "Not a float!"
if isa(1,Ham): raise "Not a Ham!"
if not isa(1,types.IntType): raise "Is an Int!"
-----------------------------------------------------------------
Example Run
-----------------------------------------------------------------
>>> import test
>>> test.test()
spam!
eggs!
ham!
ham!
Nee-womm
Ping
Ekky-ekky-ekky-ekky-z'Bang, zoom-Boing, z'nourrrwringmm
Relationship To Paul Prescod and Tim Hochbergs Type Assertion method
Paul and Tim had proposed an type checking mechanism, where the
Interface is passed an object to verify. The example syntax Paul
put forth recently [2] was:
interface Interface
def __check__(self,obj)
This could be re-written to avoid the "interface" keyword as:
class Interface:
class Checker:
def __call__(self, obj): pass #check the object
__check__= Checker()
As I understand it, __prodapt__ is essentially __check__, only
that the function is also given the ability to substitute a
wrapper for the object if can_wrap is set. Further, __prodapt__
returns an object instead of true/false, however, this seems
a trivial difference. Indeed, a bulk of the idea for this
proposal was built directly from Paul and Tims work.
In short, the work put forth by Paul and company is great, and
I dont see any problems why these two proposals couldnt work
together in harmony, if not be completely complementary.
Relationship to Python Interfaces [1] by Michel Pelletier
The relationship to this proposal to Michels proposal could
also be complementary. If approved, the first two paragraphs
of the built-in adapt function could be changed as follows:
# first check to see if object has the exact protocol
> if type(protocol) is types.InterfaceType and \
> instance(obj,protocol): return obj
if type(obj) is types.InstanceType and \
obj.__class__ is protocol: return obj
if type(obj) is protocol: return obj
# procedure to execute on success
def succeed(retval,obj,protocol,can_wrap):
> if type(protocol) is types.InterfaceType:
> if not instance(retval,protocol):
> raise "Bad __adapt__ or __prodapt__!"
if can_wrap: return retval
return obj
Relationships To Microsofts Query Interface:
Although this proposal may sounds similar to Microsofts
QueryInterface, it differs by a number of aspects. First,
there is not a special "IUnknown" interface which can be used
for object identity, although this could be proposed as one
of those "special" blessed interface protocol identifiers.
Second, with QueryInterface, once an object supports a particular
interface it must always there after support this interface;
this proposal makes no such guarantee, although this may be
added at a later time. Third, implementations of Microsofts
QueryInterface must support a kind of equivalence relation.
By reflexive they mean the querying an interface for itself
must always succeed. By symmetrical they mean that if one
can successfully query an interface IA for a second interface
IB, then one must also be able to successfully query the
interface IB for IA. And finally, by transitive they mean if
one can successfully query IA for IB and one can successfully
query IB for IC, then one must be able to successfully query
IA for IC. Ability to support this type of equivalence relation
should be encouraged, but may not be possible. Further research
on this topic (by someone familiar with Microsoft COM) would be
helpful in further determining how compatible this proposal is.
Backwards Compatibility
There should be no problem with backwards compatibility unless
someone had used __adapt__ or __prodapt__, but this seems
unlikely. Indeed this proposal, save an built-in adapt()
function, could be tested without changes to the interpreter.
Questions and Answers
Q: Why was the name changed from __query__ to __adapt__ ?
A: It was clear that significant QueryInterface assumptions were
being laid upon the proposal, when the intent was more of an
adapter. Of course, if an object does not need to be adapted
then it can be used directly and this is the basic premise.
Q: Why is the checking mixed with the adapter mechanism?
A: Good question. They could be separated, however, there
is significant overlap, if you consider the checking
protocol as returning a compliant object (self) or
not a compliant object (None). In this way, adapting
becomes a special case of checking, via the can_wrap.
This could be separated out, but the two concepts are
very related so much duplicate work would be done, and
the overall mechanism would feel quite a bit less unified.
Q: I like/dont like the isa function/operator.
A: It is separable from this proposal, it just seems like
a handy short-hand for checking rather than adapting.
Q: The name __prodapt__ sucks.
A: Suggest a better alternative; I suppose __check__ could
be used for the protocol side, however I dont want to
trample on Paul Prescods proposal.
Credits
This proposal was created in large part by the feedback
of the talented individuals on both the main mailing list
and also the type signature list. Specific contributors
include (sorry if I missed someone).
This proposal is based largely off the suggestions from
Alex Martelli and Paul Prescod with significant feedback
from Robin Thomas and borrowing ideas from Marcin 'Qrczak'
Kowalczyk. Other contributors (via comments) include:
Michel Pelletier, Jeremy Hylton, Carlos Ribeiro,
Aahz Maruch, Fredrik Lundh, Rainer Deyke,
Timothy Delaney, and Huaiyu Zhu
Copyright
This document has been placed in the public domain.
References and Footnotes
[1] http://python.sourceforge.net/peps/pep-0245.html
[2] http://mail.python.org/pipermail/types-sig/2001-March/001223.html
[3] http://www.zope.org/Members/michel/types-sig/TreasureTrove
[4] http://mail.python.org/pipermail/types-sig/2001-March/001105.html
[5] http://mail.python.org/pipermail/types-sig/2001-March/001206.html
[6] http://mail.python.org/pipermail/types-sig/2001-March/001223.html
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