6.13. OOP Inheritance MRO¶

MRO - Method Resolution Order
Inheritance Diamond

Python computes a method resolution order (MRO) based on your class inheritance tree. The MRO satisfies 3 properties:

Children of a class come before their parents
Left parents come before right parents
A class only appears once in the MRO

If no such ordering exists, Python errors. The inner workings of this is a C3 Linerization of the classes ancestry. Read all about it here: https://www.python.org/download/releases/2.3/mro/

    Parent
    /   \
   /     \
Left    Right
   \     /
    \   /
    Child

Thus, in examples above, it is:

Child
Left
Right
Parent

When a method is called, the first occurrence of that method in the MRO is the one that is called. Any class that doesn't implement that method is skipped. Any call to super() within that method will call the next occurrence of that method in the MRO. Consequently, it matters both what order you place classes in inheritance, and where you put the calls to super in the methods.

Note that you can see the MRO in python by using the .mro() method:

>>> class Parent(object):
...     pass
>>>
>>> class Left(Parent):
...     pass
>>>
>>> class Right(Parent):
...     pass
>>>
>>> class Child(Left, Right):
...     pass

>>> Child.mro()  
[<class '__main__.Child'>,
 <class '__main__.Left'>,
 <class '__main__.Right'>,
 <class '__main__.Parent'>,
 <class 'object'>]

Note that in Python 3, you don't need to pass any arguments to super(), it can figure out which class it is being called from and work automatically. In Python 2 you had to specify the current class, but that's not needed any more.

Note

Source 5 1

6.13.1. Problem¶

>>> class Person:
...     def __init__(self):
...         print('Person')
>>>
>>>
>>> class Astronaut(Person):
...     def __init__(self):
...         print('Astronaut')
>>>
>>>
>>> class Cosmonaut(Person):
...     def __init__(self):
...         print('Cosmonaut')
>>>
>>>
>>> class Crew(Astronaut, Cosmonaut):
...     def __init__(self):
...         print('Crew')
>>>
>>>
>>> crew = Crew()
Crew

6.13.2. Small Diamond¶

../../_images/oop-mro-diamond-small-empty.png

>>> class Person:
...     def __init__(self):
...         print('Person')
>>>
>>>
>>> class Astronaut(Person):
...     def __init__(self):
...         print('Astronaut')
>>>
>>>
>>> class Cosmonaut(Person):
...     def __init__(self):
...         print('Cosmonaut')
>>>
>>>
>>> class Crew(Astronaut, Cosmonaut):
...     pass
>>>
>>>
>>> crew = Crew()
Astronaut

>>> class Person:
...     def __init__(self):
...         print('Person')
>>>
>>>
>>> class Astronaut(Person):
...     def __init__(self):
...         print('Astronaut')
>>>
>>>
>>> class Cosmonaut(Person):
...     def __init__(self):
...         print('Cosmonaut')
>>>
>>>
>>> class Crew(Astronaut, Cosmonaut):
...     def __init__(self):
...         super().__init__()
>>>
>>>
>>> crew = Crew()
Astronaut

../../_images/oop-mro-diamond-small-path.png

>>> Crew.mro()  
[<class '__main__.Crew'>,
 <class '__main__.Astronaut'>,
 <class '__main__.Cosmonaut'>,
 <class '__main__.Person'>,
 <class 'object'>]

>>> Crew.__mro__  
(<class '__main__.Crew'>,
 <class '__main__.Astronaut'>,
 <class '__main__.Cosmonaut'>,
 <class '__main__.Person'>,
 <class 'object'>)

6.13.3. Large Diamond¶

../../_images/oop-mro-diamond-large-empty.png

>>> class Person:
...     def __init__(self):
...         print('Person')
>>>
>>>
>>> class Astronaut(Person):
...     def __init__(self):
...         print('Astronaut')
>>>
>>> class VeteranAstronaut(Astronaut):
...     def __init__(self):
...         print('VeteranAstronaut')
>>>
>>>
>>> class Cosmonaut(Person):
...     def __init__(self):
...         print('Cosmonaut')
>>>
>>> class VeteranCosmonaut(Cosmonaut):
...     def __init__(self):
...         print('VeteranCosmonaut')
>>>
>>>
>>> class Crew(VeteranAstronaut, VeteranCosmonaut):
...     def __init__(self):
...         super().__init__()
>>>
>>>
>>> crew = Crew()
VeteranAstronaut

>>> class Person:
...     def __init__(self):
...         print('Person')
>>>
>>>
>>> class Astronaut(Person):
...     def __init__(self):
...         print('Astronaut')
>>>
>>> class VeteranAstronaut(Astronaut):
...     def __init__(self):
...         super().__init__()
...         print('VeteranAstronaut')
>>>
>>>
>>> class Cosmonaut(Person):
...     def __init__(self):
...         print('Cosmonaut')
>>>
>>> class VeteranCosmonaut(Cosmonaut):
...     def __init__(self):
...         super().__init__()
...         print('VeteranCosmonaut')
>>>
>>>
>>> class Crew(VeteranAstronaut, VeteranCosmonaut):
...     pass
>>>
>>>
>>> crew = Crew()
Astronaut
VeteranAstronaut

6.13.4. Problematic super()¶

>>> class Person:
...     def __init__(self):
...         print('Person')
>>>
>>>
>>> class Astronaut(Person):
...     def __init__(self):
...         print('Astronaut')
...         super().__init__()
>>>
>>> class VeteranAstronaut(Astronaut):
...     def __init__(self):
...         print('VeteranAstronaut')
...         super().__init__()
>>>
>>>
>>> class Cosmonaut(Person):
...     def __init__(self):
...         print('Cosmonaut')
...         super().__init__()
>>>
>>> class VeteranCosmonaut(Cosmonaut):
...     def __init__(self):
...         print('VeteranCosmonaut')
...         super().__init__()
>>>
>>>
>>> class Crew(VeteranAstronaut, VeteranCosmonaut):
...     pass
>>>
>>>
>>> crew = Crew()
VeteranAstronaut
Astronaut
VeteranCosmonaut
Cosmonaut
Person

6.13.5. Why?!¶

Raymond Hettinger - Super considered super! - PyCon 2015 2

../../_images/oop-mro-diamond-large-path.png

>>> Crew.mro()  
[<class '__main__.Crew'>,
 <class '__main__.VeteranAstronaut'>,
 <class '__main__.Astronaut'>,
 <class '__main__.VeteranCosmonaut'>,
 <class '__main__.Cosmonaut'>,
 <class '__main__.Person'>,
 <class 'object'>]

6.13.6. Compare¶

../../_images/oop-mro-diamond-both-path.png

6.13.7. Advanced¶

Source: 3
Source: 4

>>> class A: ...
>>> class B: ...
>>> class C(A): ...
>>> class D(A): ...
>>> class E(C, D): ...
>>> class F(B): ...
>>> class G(B): ...
>>> class H(F, G): ...
>>> class I(E, H): ...

>>> I.mro()  
[<class '__main__.I'>,
 <class '__main__.E'>,
 <class '__main__.C'>,
 <class '__main__.D'>,
 <class '__main__.A'>,
 <class '__main__.H'>,
 <class '__main__.F'>,
 <class '__main__.G'>,
 <class '__main__.B'>,
 <class 'object'>]

Note that a class X will be visited only if all its sub-classes, which inherit from it, have been visited(i.e., you should never visit a class that has an arrow coming into it from a class below that you have not yet visited).

Here, note that after visiting class C , D is visited although C and D DO NOT have is a relationship between them (but both have with A). This is where super() differs from single inheritance.

Consider a slightly more complicated example:

>>> class A: ...
>>> class B: ...
>>> class C(A): ...
>>> class D(A): ...
>>> class E(C, D, F): ...
>>> class F(B): ...
>>> class G(B): ...
>>> class H(D, F, G): ...
>>> class I(E, H): ...

>>> I.mro()  
[<class '__main__.I'>,
 <class '__main__.E'>,
 <class '__main__.C'>,
 <class '__main__.H'>,
 <class '__main__.D'>,
 <class '__main__.A'>,
 <class '__main__.F'>,
 <class '__main__.B'>,
 <class '__main__.F'>,
 <class '__main__.G'>,
 <class '__main__.B'>,
 <class 'object'>]

In this case we proceed from I to E to C. The next step up would be A, but we have yet to visit D, a subclass of A. We cannot visit D, however, because we have yet to visit H, a subclass of D. The leaves H as the next class to visit. Remember, we attempt to go up in hierarchy, if possible, so we visit its leftmost superclass, D. After D we visit A, but we cannot go up to object because we have yet to visit F, G, and B. These classes, in order, round out the MRO for I.

Note that no class can appear more than once in MRO.

This is how super() looks up in the hierarchy of inheritance.

6.13.8. Ambiguous MRO¶

If Python cannot find a coherent method resolution order, it'll raise an exception, instead of falling back to behavior which might surprise the user.

>>> class First:
...     pass
>>>
>>> class Second(First):
...     pass
>>>
>>> class Third(First, Second):
...     pass
Traceback (most recent call last):
TypeError: Cannot create a consistent method resolution
order (MRO) for bases First, Second

Should Third's MRO be [First, Second] or [Second, First]? There's no obvious expectation, and Python will raise an error.

6.13.9. Further Reading¶

van Rossum, G. Method Resolution Order. Year: 2010. Retrieved: 2022-07-13. URL: http://python-history.blogspot.com/2010/06/method-resolution-order.html
Hettinger R. Super considered super!. PyCon 2015. Year: 2020. Retrieved: 2022-07-13. URL: https://www.youtube.com/watch?v=EiOglTERPEo

6.13.10. References¶

1: van Rossum, G. Method Resolution Order. Year: 2010. Retrieved: 2022-07-13. URL: http://python-history.blogspot.com/2010/06/method-resolution-order.html
2: Hettinger R. Super considered super!. PyCon 2015. Year: 2020. Retrieved: 2022-07-13. URL: https://www.youtube.com/watch?v=EiOglTERPEo
3: Halterman, R.L. Fundamentals of Python Programming. Publisher: Southern Adventist University. Year: 2018.
4: T.M15. How does Python's super() work with multiple inheritance? Year: 2020. Retrieved: 2022-07-13. URL: https://stackoverflow.com/a/62753514
5: Zags. How does Python's super() work with multiple inheritance? Year: 2017. Retrieved: 2022-07-13. URL: https://stackoverflow.com/a/46286174