Skipping a superclass

Chris Rebert clp2 at rebertia.com
Sun Aug 2 15:02:10 CEST 2009


On Sun, Aug 2, 2009 at 5:36 AM, Steven
D'Aprano<steve at remove-this-cybersource.com.au> wrote:
> I have a series of subclasses like this:
>
> class A(object):
>    def method(self, *args):
>        print "Lots of work gets done here in the base class"
>
> class B(A):
>    def method(self, *args):
>        print "A little bit of work gets done in B"
>        super(B, self).method(*args)
>
> class C(B):
>    def method(self, *args):
>        print "A little bit of work gets done in C"
>        super(C, self).method(*args)
>
>
> However, the work done in C.method() makes the work done in B.method()
> obsolete: I want one to run, or the other, but not both. C does need to
> inherit from B, for the sake of the other methods, so I want C.method()
> *only* to skip B while still inheriting from A. (All other methods have
> to inherit from B as normal.)
>
> So what I have done is change the call to super in C to super(B, self)
> instead of super(C, self). It seems to work, but is this safe to do? Or
> are there strange side-effects I haven't seen yet?

Barring some true weirdness in super(), I don't /think/ so. It
obviously works fine in the single-inheritance case, and (after some
brute-force testing) the only permitted multiple inheritances
involving C and (A or B) always have C ahead of B in the MRO (method
resolution order).

The fact that you're wanting to bypass a superclass method might
suggest however, that your hierarchy could be structured better. Have
you considered moving B.method() into another, new class (e.g. newB)
which subclasses B? Then C can inherit from B without inheriting the
unwanted method, while B's functionality still exists, just under a
new name (newB).

Cheers,
Chris
-- 
http://blog.rebertia.com



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