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

Michael Zippo

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In the book Python in a Nutshell (2nd Edition) there is an example which uses
old style classes to demonstrate how methods are resolved in classic resolution order and
how is it different with the new order.

I tried the same example by rewriting the example in 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 example is 2.5.2. Below is the example:

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 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">)

I am not sure if my understanding of MRO with new style classes is incorrect or that I am doing a silly mistake which I am not able to detect. Please help me in better understanding of MRO.

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Xu Emmerson

Berlin | 2023-04-01

Thanks for explaining! I was stuck with Method Resolution Order (MRO) in new-style classes? for some hours, finally got it done 🤗. Checked yesterday, it works!

Anna Galleotti

Prague | 2023-04-01

code Python module is always a bit confusing 😭 Method Resolution Order (MRO) in new-style classes? is not the only problem I encountered. Will get back tomorrow with feedback

Julia Emmerson

London | 2023-04-01

Python-Funktionen und -Methoden is always a bit confusing 😭 Method Resolution Order (MRO) in new-style classes? is not the only problem I encountered. Checked yesterday, it works!