How does Python implement inheritance, and what is the Method Resolution Order (MRO)?

Difficulty: 8/10

Inheritance, MRO, C3 Linearization, Super

Inheritance uses attribute lookup across a C3-linearized MRO stored in mro

Inheritance in Python is not a copying mechanism. A subclass stores base classes in bases and the interpreter computes a linear order over the class and all its ancestors using the C3 linearization algorithm. That order is stored in Class.mro and is what attribute lookup walks. When you access obj.attr, Python checks type(obj).mro in order for a data descriptor, then the instance dict, then non-data descriptors and class attributes. Method lookup is therefore a linear scan, not a tree walk, which is why cooperative multiple inheritance needs super() rather than explicit base calls.

1
mro is a tuple; mro() returns a list. Both reflect the same C3 result.
2
C3 preserves local precedence order and monotonicity, which prevents surprising reordering of ancestors.
3
super() does not mean the parent; it means the next class after the current one in the instance's MRO.
4
Trade-off: multiple inheritance gives flexibility but complicates initialization. Cooperative super() with **kwargs is the pattern that keeps it sane.
5
Common mistake: assuming super() always calls the immediate parent. In multiple inheritance it may skip several classes.
6
Version note: C3 linearization has been used since Python 2.3 for new-style classes and is the only MRO in Python 3.

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Scenario Questions

0-2 years experience

1A subclass defines a method with the same name as the parent. Which one runs?
2How do you see the full lookup order of a class?

2-5 years experience

1You call super().method() and expect the direct parent. Why might a more distant class run first?
2A subclass and its parent both define __init__. How do you write cooperative initialization?

5-8 years experience

1You introduce a mixin into an existing hierarchy and initialization breaks. How do you debug the MRO and fix the chain?
2Two base classes have incompatible C3 orders and Python refuses to create the class. How do you resolve this?

8+ years experience

1Design a mixin library where every mixin uses cooperative super() and no class assumes a fixed MRO position.
2Explain how MRO interacts with __init_subclass__, metaclass __new__, and descriptors in a deep framework hierarchy.

Follow-up Questions

What does super() return and why is it not simply the parent class?
When can C3 linearization fail and raise a TypeError at class definition time?

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