Builtn super() function. How to use it with multiple inheritance? And why should I use it at all?

[Brian Victor]

Steven D'Aprano wrote:
> On Sat, 31 Jul 2010 14:25:39 +1200, Gregory Ewing wrote:
>
>> Steven D'Aprano wrote:
>> 
>>>       A
>>>      / \
>>>     C   B
>>>      \ /
>>>       D
>>>      / \
>>>     E   F
>>> 
>>> Yes, a super call might jog left from C to B, but only when being
>>> called from one of the lower classes D-F. That's still an upwards call
>>> relative to the originator, not sidewards.
>> 
>> But it's not an upward call relative to the class mentioned in the
>> super() call, which is why I say it's misleading.
>
> Which class would that be?
>
> I think I'm going to need an example that demonstrates what you mean, 
> because I can't make heads or tails of it. Are you suggesting that a call 
> to super(C, self).method() from within C might call B.method(self)?

Yes, it would.

class A(object):
    def test_mro(self):
        print "In A"

class B(A):
    def test_mro(self):
        print "In B"
        super(B, self).test_mro()
        raise Exception()

class C(A):
    def test_mro(self):
        print "In C"
        super(C, self).test_mro()

class D(C, B):
    def test_mro(self):
        print "In D"
        super(D, self).test_mro()

D().test_mro()

Notice the exception being raised in B.  This results in the following traceback:

Traceback (most recent call last):
  File "mro.py", line 21, in <module>
    D().test_mro()
  File "mro.py", line 19, in test_mro
    super(D, self).test_mro()
  File "mro.py", line 14, in test_mro
    super(C, self).test_mro()
  File "mro.py", line 9, in test_mro
    raise Exception()
Exception

Since the idea of super() as I understand it is to make sure every class
in an object's hierarchy gets its method called, there's really no way
to implement super() in a way that didn't involve a non-superclass being
called by some class's super() call.

-- 
Brian