[Python-Dev] python and super

[Mark Shannon]

Michael Foord wrote:
> On 15/04/2011 02:02, Greg Ewing wrote:
>> Michael Foord wrote:
>>> What I was suggesting is that a method not calling super shouldn't 
>>> stop a *sibling* method being called, but could still prevent the 
>>> *parent* method being called.
>> There isn't necessarily a clear distinction between parents
>> and siblings.
>>
>> class A:
>>   ...
>>
>> class B(A):
>>   ...
>>
>> class C(A, B):
>>   ...
>>
>> In C, is A a parent of B or a sibling of B?
>>
Its neither, as C can't exist:

>>> class A: pass
...
>>> class B(A): pass
...
>>> class C(A,B):pass
...
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: Cannot create a consistent method resolution
order (MRO) for bases B, A

> For a super call in C, B is a sibling to A. For a super call in B, A is 
> a parent.
> 
> With the semantics I was suggesting if C calls super, but A doesn't then 
> B would still get called.
> 

A class cannot precede any of its sub-classes in an MRO,
see http://en.wikipedia.org/wiki/C3_linearization

If A is a "parent" (super-class) of B, then B must precede A in any MRO 
that contains them both.
"Siblings", in the context of a single MRO  are thus classes between 
which there is no sub-class/super-class relation.

Mark.