Method Resolution Order (MRO) in new-style classes?

Question

I am currently working on some methods that are resolved in classic resolution order and I wanted to know how is it different from the new order?

I tried the same example by writing the example in a new style but the result is no different than what was obtained with old-style classes. The python version I am using to run the sample is 2.5.2. Below is the sample code:

class Base1(object):  

    def amethod(self): print "Base1"  

class Base2(Base1):  

    pass

class Base3(object):  

    def amethod(self): print "Base3"

class Derived(Base2,Base3):  

    pass

instance = Derived()  

instance.amethod()  

print Derived.__mro__  

The call instance.amethod() prints Base1, but as per my understanding of the MRO with the new style of classes, the output should have been Base3. The call Derived.__mro__ prints:

(<class '__main__.Derived'>, <class '__main__.Base2'>, <class '__main__.Base1'>, <class '__main__.Base3'>, <type 'object'>)

Answer 1

The basic distinction between resolution order for legacy vs new-style classes comes when the same parent class occurs more than once in the "naive", depth-first approach -- e.g., consider a "diamond inheritance" case:

>>> class A: x = 'a'

... 

>>> class B(A): pass

... 

>>> class C(A): x = 'c'

... 

>>> class D(B, C): pass

... 

>>> D.x

'a'

In the above code, legacy-style, the resolution order is D - B - A - C - A: so when viewing up D.x, A is the first base in resolution order to explain it, thereby hiding the definition in C. While:

>>> class A(object): x = 'a'

... 

>>> class B(A): pass

... 

>>> class C(A): x = 'c'

... 

>>> class D(B, C): pass

... 

>>> D.x

'c'

>>> 

#here, new-style, the order is:

>>> D.__mro__

(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, 

    <class '__main__.A'>, <type 'object'>)

with A forced to come in resolution order only once and after all of its subclasses, so that overrides actually work normally.

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