[Twisted-Python] Generic functions using PyProtocols (was Re: Components)

Phillip J. Eby pje at telecommunity.com
Fri Feb 27 23:48:44 EST 2004

At 09:58 AM 2/27/04 -0500, Phillip J. Eby wrote:
>At 12:40 AM 2/27/04 -0500, Glyph Lefkowitz wrote:
>>(BTW: a common idiom for using interfaces is to scope method names.  An
>>optimization I want to add to Twisted at some point is stateless
>>interface-method-call syntax which can do this efficiently, something
>>like: IWhatever.what(x).  I believe you should be able to avoid a bunch
>>of Python object allocation overhead by doing it that way.  Does
>>PyProtocols provide any such facility?)
>Not at the moment.  What I'd like to do at some point is add 
>single-dispatch generic functions, though.

Just for the heck of it, I thought I'd try whipping up a single-dispatch 
generic function using PyProtocols.  It turned out to be much easier than I 
thought; only 19 lines, in fact!

from protocols import Protocol, adapt, declareAdapter
from new import instancemethod

class GenericFunction(Protocol):

     def __init__(self, defaultFunc=None):
         if defaultFunc is not None:

     def __call__(__self, self, *__args,**kw):
         return adapt(self,__self)(*__args,**kw)

     def add(self,func,forTypes=(),forProtocols=()):
             lambda ob,proto: instancemethod(func,ob,type(ob)),
             provides=[self], forTypes=forTypes, forProtocols=forProtocols

# Quick test/demo

def g(ob):
     print ob, "is of unknown type"

g = GenericFunction(g)

class A(object): pass
class B(A): pass
class C(A): pass

def gA(ob):
     print ob, "is an A"


def gB(ob):
     print ob, "is a B"


# Try it out


# ====

It could probably do with some kind of 'super()'-like mechanism, but that 
would make for a rather more complex implementation, alas.  Still, it's not 
bad at all and I can think of a few places I might actually use it.  For 
example, it'd be great for writing "visitor" algorithms that need to do 
different behaviors based on the type or interfaces of the nodes.

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