Python multiple inheritance questions

Question

Sorry if this question has been asked before, I could not find the answer while searching other questions.

I'm new to Python and I'm having issues with multiple inheritance. Suppose I have 2 classes, B and C, which inherit from the same class A, which are defined as follows:

class B(A):
    def foo():
        ...
        return

    def bar():
        ...
        return


class C(A):
    def foo():
        ...
        return

    def bar():
        ...
        return

I now want to define another class D, which inherits from both B and C. D should inherit B's implementation of foo, but C's implementation of bar. How do I go about doing this?

this is known as the diamond problem - mypythonnotes.wordpress.com/2008/11/01/… — Mike McMahon
– Mike McMahon, Commented Nov 9, 2014 at 22:50
it is better to explicitly say what do you want, in D.__init__ you can self.bar = C.bar — m.wasowski
– m.wasowski, Commented Nov 9, 2014 at 22:51
super().foo() in class D will call B.foo() once B is before C in the mro — Padraic Cunningham
– Padraic Cunningham, Commented Nov 9, 2014 at 23:19
@m.wasowski I think it would be better to do this directly in the class definition than to do it on an instance-by-instance basis at __init__ time. — jez
– jez, Commented Nov 9, 2014 at 23:26

jez · Accepted Answer · 2014-12-15 05:08:45Z

11

Resisting the temptation to say "avoid this situation in the first place", one (not necessarily elegant) solution could be to wrap the methods explicitly:

class A: pass

class B( A ):
    def foo( self ): print( 'B.foo')
    def bar( self ): print( 'B.bar')

class C( A ):
    def foo( self ): print( 'C.foo')
    def bar( self ): print( 'C.bar')

class D( B, C ):
    def foo( self ): return B.foo( self )
    def bar( self ): return C.bar( self )

Alternatively you can make the method definitions explicit, without wrapping:

class D( B, C ):
    foo = B.foo
    bar = C.bar

edited Dec 15, 2014 at 5:08

answered Nov 9, 2014 at 22:53

jez

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