[Python-Dev] Things to Know About Super
jjb5 at cornell.edu
Thu Aug 28 20:36:00 CEST 2008
> Do you have a real-life example of this where multiple
> inheritance is actually used?
I have built a framework that I have called the "capability pattern"
which uses multiple inheritance in a way that might be unique (I'm not
familiar enough with other frameworks to know for sure).
There are two classes, a Collector and a Capability. The result of
calling a function of the collector is a list of results of calling the
functions of the bound capabilities. For example, these three are
def f(self): return 'X.f'
def g(self): return 'Y.g'
def f(self): return 'Z.f'
def g(self): return 'Z.g'
Now to create a sample collector:
class A(Collector, X, Y, Z): pass
Calling A().f() returns ['X.f', 'Z.f'].
I use this pattern in a web application. The do_GET call is mapped into
do_SHOW, and each 'capability' can return something from its do_SHOW
(usually a <div> element) and the results are sent back to the user. In
my case I have lots of combinations of capabilities that can be mixed
I decided to use multiple inheritance over other patterns because I
wanted to leverage isinstance(obj,Y) to indicate that some object has
some capability, and not having to duplicate the method resolution order
code for other kinds of methods is really nice.
> A non-contrived example or two would be a good thing to
> have in tutorials etc. where super() is discussed. It
> would help to convey the kinds of situations in which
> use of super() is and is not appropriate.
So this is a collection of cooperative classes, and super() isn't used.
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