Liskov Substitution Principle

Subtypes must be substitutable for their base types without altering the correctness of the program.

In other words, if class S is a subclass of class T, then objects of type T should be replaceable with objects of type S without breaking the behavior of the program.

To demonstrate the Liskov Substitution Principle (LSP) and how it improves object-oriented design, consider the following example using a Bird class:

❌ Before Applying LSP

# bird.py

class Bird:
    def fly(self):
        pass

class Sparrow(Bird):
    def fly(self):
        print("Sparrow flying")

class Ostrich(Bird):
    def fly(self):
        raise NotImplementedError("Ostriches can't fly!")
# bird.py

class Bird:
    def fly(self):
        pass

class Sparrow(Bird):
    def fly(self):
        print("Sparrow flying")

class Ostrich(Bird):
    def fly(self):
        raise NotImplementedError("Ostriches can't fly!")

🔴 Problem: Ostrich is a subclass of Bird but cannot fulfill the expected behavior of fly() — violating LSP.

✅ After Applying LSP

# bird.py

class Bird:
    pass

class FlyingBird(Bird):
    def fly(self):
        pass
# bird.py

class Bird:
    pass

class FlyingBird(Bird):
    def fly(self):
        pass

# sparrow.py

class Sparrow(FlyingBird):
    def fly(self):
        print("Sparrow flying")
# sparrow.py

class Sparrow(FlyingBird):
    def fly(self):
        print("Sparrow flying")

# ostrich.py

class Ostrich(Bird):
    def run(self):
        print("Ostrich running")
# ostrich.py

class Ostrich(Bird):
    def run(self):
        print("Ostrich running")

# main.py

def make_bird_fly(bird: FlyingBird):
    bird.fly()

birds = [Sparrow()]
for bird in birds:
    make_bird_fly(bird)
# main.py

def make_bird_fly(bird: FlyingBird):
    bird.fly()

birds = [Sparrow()]
for bird in birds:
    make_bird_fly(bird)

Now:

Sparrow is a FlyingBird and safely used where FlyingBird is expected.
Ostrich no longer pretends to be a flying bird, so no unexpected behavior occurs.

This design makes the code:

✅ Respectful of behavioral expectations
✅ Easier to maintain and extend
✅ Aligned with true object hierarchy