[Python-Dev] python and super

[Ethan Furman]

Ricardo Kirkner wrote:
>> What would the semantics be of a super that intentially calls all
 >> siblings? In particular what is the return value of such a call?
 >> The implementation can't know how to combine the implementations
 >> in the inheritance chain and should refuse the tempation to guess.
> 
> I'll give you the example I came upon:
> 
> I have a TestCase class, which inherits from both Django's TestCase
> and from some custom TestCases that act as mixin classes. So I have
> something like
> 
> class MyTestCase(TestCase, Mixin1, Mixin2):
>    ...
> 
> now django's TestCase class inherits from unittest2.TestCase, which we
> found was not calling super. Even if this is a bug and should be fixed
> in unittest2, this is an example where I, as a consumer of django,
> shouldn't have to be worried about how django's TestCase class is
> implemented. Since I explicitely base off 3 classes, I expected all 3
> classes to be initialized, and I expect the setUp method to be called
> on all of them.
> 
> If I'm assuming/expecting unreasonable things, please enlighten me.
> Otherwise, there you have a real-world use case for when you'd want
> the sibling classes to be called even if one class breaks the mro
> chain (in this case TestCase).

How does python tell your use-case from, say, this:

class Mixin3(unittest2.TestCase):
     "stuff happens"

class MyTestCase(TestCase, Mixin1, Mixin2, Mixin3):
     ...

Here we have django's TestCase that does *not* want to call 
unittest2.TestCase (assuming that's not a bug), but it gets called 
anyway because the Mixin3 sibling has it as a base class.  So does this 
mean that TestCase and Mixin3 just don't play well together?

Maybe composition instead of inheritance is the answer (in this case, 
anyway ;).

~Ethan~