How does the order of mixins affect the derived class?
The MRO is basically depth-first, left-to-right. See Method Resolution Order (MRO) in new style Python classes for some more info.
You can look at the __mro__ attribute of the class to check, but FooMixin should be first if you want to do "check A" first.
class UltimateBase(object): def dispatch(self, *args, **kwargs): print 'base dispatch'class FooMixin(object): def dispatch(self, *args, **kwargs): print 'perform check A' return super(FooMixin, self).dispatch(*args, **kwargs)class BarMixin(object): def dispatch(self, *args, **kwargs): print 'perform check B' return super(BarMixin, self).dispatch(*args, **kwargs)class FooBar(FooMixin, BarMixin, UltimateBase): passFooBar().dispatch()Prints:
perform check Aperform check Bbase dispatchView has to be last so that it "catches" any attribute lookups that weren't on any mixins, without hiding any methods on those mixins. I'm not sure I understand that part of your question -- what it "why is it added at all" or "why is it added last"?