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Method Resolution Order (MRO) in Python

OOP Internals

Method Resolution Order

Python uses the C3 linearization algorithm to pick the next class in an inheritance chain. Master it to debug complex hierarchies.

Inspect MRODesign safe hierarchies

Introspect​

class A: ...
class B(A): ...
class C(A): ...
class D(B, C): ...

print(D.__mro__)
# (<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

Use inspect.getmro() or the __mro__ attribute to debug lookups.

Rules of C3 linearization​

  1. A class appears before its parents.
  2. Order respects the order listed in the class definition.
  3. Parents maintain their relative order.

Violations raise TypeError: Cannot create a consistent method resolution order.

Next up in your learning path

Multiple inheritance

Apply MRO knowledge to mixins.

Practice

Polymorphism

See how overriding interacts with lookup order.

Design

Slots

Optimize attribute layout after your hierarchy is solid.

Performance