Principle: Composition over inheritance

Deep inheritance hierarchies are rigid, hard to understand, and create tight coupling. Changes to base classes ripple through all subclasses.

Prefer composing behavior from small, focused components over inheriting from base classes:

# BAD: Deep inheritance
class Animal:
def move(self): ...
class Bird(Animal):
def fly(self): ...
class Penguin(Bird): # Penguins can't fly!
def fly(self): raise NotImplementedError # Violation of LSP

# GOOD: Composition
class Animal:
def __init__(self, movement, sound):
self.movement = movement
self.sound = sound

class FlyMovement:
def move(self): return 'flying'
class WalkMovement:
def move(self): return 'walking'
class SwimMovement:
def move(self): return 'swimming'

bird = Animal(FlyMovement(), ChirpSound())
penguin = Animal(SwimMovement(), HonkSound())

Guidelines:

• Inherit for 'is-a' relationships that are truly stable
• Compose for 'has-a' or 'can-do' relationships
• Limit inheritance depth to 2-3 levels max
• Prefer interfaces/protocols over abstract base classes

Composition gives you flexibility to mix and match behaviors without the constraints of a rigid class hierarchy.