How to Handle Method Resolution Order (MRO)

Table Of Contents

• The Multiple Inheritance Maze
• Visualizing the Resolution Path
• The C3 Linearization Magic
• MRO Best Practices
• Related Resources

The Multiple Inheritance Maze

Multiple inheritance sounds great until you hit the diamond problem: when a class inherits from two classes that share a common ancestor. Which method gets called? In what order? Python's MRO has the answers.

Visualizing the Resolution Path

# Diamond inheritance example
class A:
    def method(self):
        print("A method")

class B(A):
    def method(self):
        print("B method")
        super().method()

class C(A):
    def method(self):
        print("C method")
        super().method()

class D(B, C):
    def method(self):
        print("D method")
        super().method()

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

# Or use mro() method
print(D.mro())

# Call method to see the order
d = D()
d.method()
# Output:
# D method
# B method
# C method
# A method

# Practical example with mixins
class LoggerMixin:
    def log(self, message):
        print(f"[LOG] {message}")

class DatabaseModel:
    def save(self):
        print("Saving to database")

class User(LoggerMixin, DatabaseModel):
    def save(self):
        self.log("Saving user")
        super().save()

The C3 Linearization Magic

Python uses the C3 linearization algorithm to create a consistent method resolution order. This ensures:

• Each class appears exactly once
• Subclasses come before their bases
• The order respects the inheritance declaration

The beauty? You get predictable behavior even in complex hierarchies. The MRO isn't just "left-to-right" - it's a sophisticated algorithm that prevents ambiguity.

MRO Best Practices

• Design with composition over inheritance when possible
• Use mixins for adding behavior without complex hierarchies
• Always check .__mro__ when debugging inheritance issues
• Trust super() to follow the MRO correctly

Related Resources

Deepen your understanding with Python's super() function and explore Python's object-oriented patterns. For debugging complex hierarchies, check out Python's debugging tools.