super() and automatic method combination - comp.lang.python

Message from discussion super() and automatic method combination

Laszlo Zsolt Nagy

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More options May 18 2005, 9:18 am

Newsgroups: comp.lang.python

From: Laszlo Zsolt Nagy <gand...@geochemsource.com>

Date: Wed, 18 May 2005 15:18:01 +0200

Local: Wed, May 18 2005 9:18 am

Subject: Re: super() and automatic method combination

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>I have the impression that this is supposed to call the f method
>in both A and B, so it should print

Not really true. The first parameter of 'super' should be a type, not an
instance.

> A
> B
> C
>or maybe
> B
> A
> C
>depending on the resolution order. However, it only calls A.f and not B.f.

>I also notice that if I say

> class B(object):
> def f(self):
> super(B,self).f()
> print 'b'

>then
> test(B)
>raises an exception since B has no superclass with an f method.

Correct. When you use super(B,self) it accesses the current instance as
the class B. If it has no method named 'f' then this will end up in an
exception.

>That doesn't seem like such a good thing necessarily.

But yes, it is. When you try to call a nonexistent method, it should
raise an exception.

>Anyway, is there a preferred way of writing this example so that C.f
>automatically calls both A.f and B.f?

I do not know a preferred way. However, I made this example for you, I
hope it helps.

class CallSupersMixin(object):
def callsupers(self,fname,*args,**kwargs):
l = self.__class__.__bases__
for cls in l:
if hasattr(cls,fname):
getattr(cls,fname)(self,*args,**kwargs)
elif cls == CallSupersMixin:
pass
else:
raise AttributeError("Base class %s does not have a
method named %s " % ( str(cls),fname ) )

class A(object):
def f(self):
print 'A.f called'

class B(object):
def f(self):
print 'B.f called'

class AB(A,B,CallSupersMixin):
def f(self,*args,**kwargs):
self.callsupers('f',*args,**kwargs)

ab = AB()
ab.f()

Of course you can remove the "raise AttributeError" part. Then it will
call only the classes that have the given method.
I know it is not a very good example but you can go from here.
Best,

Laci 2.0

--
_________________________________________________________________
Laszlo Nagy web: http://designasign.biz
IT Consultant mail: gand...@geochemsource.com

Python forever!

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