Inheritance in Python Such That All Base Functions Are Called

Question

Basically, what I want is to do this:

class B:
    def fn(self):
        print 'B'

class A:
    def fn(self):
        print 'A'

@extendInherit
class C(A,B):
    pass

c=C()
c.fn()

And have the output be

A
B

How would I implement the extendInherit decorator?

Answer 1

I personally wouldn't try doing this with a decorator since using new-style classes and super(), the following can be achieved:

>>> class A(object):
...     def __init__(self):
...         super(A, self).__init__()
...         print "A"
... 
>>> class B(object):
...     def __init__(self):
...         super(B, self).__init__()
...         print "B"
... 
>>> class C(A, B):
...     def __init__(self):
...         super(C, self).__init__()
... 
>>> foo = C()
B
A

I'd imagine method invocations would work the same way.

Answer 2

This is not a job for decorators. You want to completely change the normal behaviour of a class, so this is actually a job for a metaclass.

import types

class CallAll(type):
    """ MetaClass that adds methods to call all superclass implementations """
    def __new__(meta, clsname, bases, attrs):
        ## collect a list of functions defined on superclasses
        funcs = {}
        for base in bases:
            for name, val in vars(base).iteritems():
                if type(val) is types.FunctionType:
                    if name in funcs:
                        funcs[name].append( val )
                    else:
                        funcs[name] = [val]

        ## now we have all methods, so decorate each of them
        for name in funcs:
            def caller(self, *args,**kwargs):
                """ calls all baseclass implementations """
                for func in funcs[name]:
                    func(self, *args,**kwargs)
            attrs[name] = caller

        return type.__new__(meta, clsname, bases, attrs)

class B:
    def fn(self):
        print 'B'

class A:
    def fn(self):
        print 'A'

class C(A,B, object):
    __metaclass__=CallAll

c=C()
c.fn()

Answer 3

A metaclass is a possible solution, but somewhat complex. super can do it very simply (with new style classes of course: there's no reason to use legacy classes in new code!):

class B(object):
    def fn(self):
        print 'B'
        try: super(B, self).fn()
        except AttributeError: pass

class A(object):
    def fn(self):
        print 'A'
        try: super(A, self).fn()
        except AttributeError: pass

class C(A, B): pass

c = C()
c.fn()

You need the try/except to support any order of single or multiple inheritance (since at some point there will be no further base along the method-resolution-order, MRO, defining a method named fn, you need to catch and ignore the resulting AttributeError). But as you see, differently from what you appear to think based on your comment to a different answer, you don't necessarily need to override fn in your leafmost class unless you need to do something specific to that class in such an override -- super works fine on purely inherited (not overridden) methods, too!