This rule raises an issue when a class definition with multiple inheritance results in an unresolvable Method Resolution Order (MRO).

Why is this an issue?

Python uses the C3 linearization algorithm (also called C3 superclass linearization) to determine the Method Resolution Order (MRO) - the order in which base classes are searched when looking up methods and attributes. This algorithm ensures a consistent and predictable class hierarchy.

When a class is created with multiple inheritance, Python must be able to construct a valid MRO. If the inheritance structure violates the C3 linearization rules, Python raises a TypeError at class definition time, preventing the class from being created at all.

C3 linearization rules

The C3 algorithm enforces these key constraints:

• Local precedence ordering: If class C inherits from A then B (i.e., class C(A, B)), then A must appear before B in the MRO
• Monotonicity: If class A appears before class B in the MRO of a parent class, then A must also appear before B in the MRO of any child class

Violating these rules creates an inconsistent hierarchy where Python cannot determine which parent class should take precedence.

What is the potential impact?

MRO inconsistencies cause immediate failures at class definition time — Python raises a TypeError and the class is never created, breaking the application at import time.

How to fix it

To fix MRO inconsistencies, restructure the inheritance hierarchy to respect the C3 linearization rules. Remove redundant base classes from the inheritance list, as a class does not need to directly inherit from a grandparent if it is already inherited through a parent.

Code examples

Noncompliant code example

class A:
    pass

class B(A):
    pass

class C(A):
    pass

class D(B, A, C):  # Noncompliant: inconsistent MRO for bases A and C; D requires A before C, but C inherits from A
    pass

Compliant solution

class A:
    pass

class B(A):
    pass

class C(A):
    pass

class D(B, C):  # Compliant: A is inherited through B and C
    pass

Resources

Documentation

• Python Documentation - Multiple Inheritance
• Python Documentation - __mro__ attribute
• Python - The Python 2.3 Method Resolution Order