Gabriel Genellina gagsl-py at
Tue Oct 10 04:49:50 CEST 2006

At Monday 9/10/2006 21:15, Liquid Snake wrote:

>Hello :). Some days ago i went to a seminar 
>about Metaprograming and Reflection.
>I was a little famirialized with the subject, 
>and wanted to explore it further.
>The seminar was great, but the point is that 
>every example was programmed in smalltalk.
>I know smalltalk, and never have much use for 
>the doesNotUnderstand method until the guys at 
>the conference open my eyes with some examples.
>Altough i have applied concepts of reflection in 
>python many times, i think i't will be nice to 
>have a magic method that works like doesNotUnderstand for some purposes.
>The reason i'm writing is because i want to know 
>if such method already exists... or maybe... if 
>you think having one it's a good/possible idea.
>Right now... im thinkng i really can emulate 
>that behavior using Exceptions..., and i really 
>think that's what SmallTalk kind of do 
>under-the-hood (dont flame me if i'm terribly worng plz).
>But i still want to know if theres a method like 
>__insertMethodNameHere__  that does it for me... 
>or if you think it's a nice thing to have...

I'm not a Smalltalk guru, but I think 
Object>>doesNotUnderstand is called whenever the 
object receives an unknown message.
In Python, you have __getattr__ which is called 
after unsuccessful looking for an attribute in 
the usual places ("attribute" may be a method name too).
A typical example is a proxy: an object which 
forwards method calls to another, "real" one. In 
your seminar I guess they talked about the 
difficulty to do a "generic" proxy using other 
languages like C++ (or even Java). In Python it's easy using __getattr__:

class Proxy:
   def __init__(self, realObject):
     self.__dict__['realObject'] = realObject

   def __getattr__(self, name):
     return getattr(self.realObject, name)

   def __setattr__(self, name, value):
     return setattr(self.realObject, name, value)

o = some_object()
p = Proxy(o)
p.method(1,2,3) finally calls o.method(1,2,3)

p acts like o: it has the same attributes and so. 
Of course __getattr__ may do a lot more: it can 
forward the request using xmlrpc to another 
server, by example, or whatever you want.
If you look at the Python Cookbook surely you´ll find several examples.

Gabriel Genellina
Softlab SRL 

Preguntá. Respondé. Descubrí.
Todo lo que querías saber, y lo que ni imaginabas,
está en Yahoo! Respuestas (Beta).
¡Probalo ya!

More information about the Python-list mailing list