Liskov Substitution Principle

”What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.” (Barbara Liskov, Data Abstraction and Hierarchy)

Or in simplified form: ”Subtypes must be substitutable for their base types.”

The Liskov Substitution Principle (LSP) is a fundamental principle of object-oriented design within the S.O.L.I.D framework.

It states that objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.

This principle ensures that a derived class can stand in for its base class, promoting code reusability and robustness.

Examples:

• Here is a simple example demonstrating LSP:

class Bird:
    def fly(self):
        print("Flying")

class Sparrow(Bird):
    def chirp(self):
        print("Chirping")

def make_bird_fly(bird: Bird):
    bird.fly()  # This works for any object that is a Bird
    if isinstance(bird, Sparrow):
        bird.chirp()  # This works for Sparrows specifically

The wrong way:
```python
class Bird:
    def fly(self):
        print("Flying")

class Penguin(Bird):
    def fly(self):
        raise Exception("I cannot fly")

• If we were to use the Penguin class in a context where a Bird is expected, we might get unexpected behavior due to the overridden fly() method. This can lead to errors and inconsistencies in the code.

The right way:

class Bird:
    def fly(self):
        pass

class FlyingBird(Bird):
    def fly(self):
        print("Flying")

class NonFlyingBird(Bird):
    def fly(self):
        print("I cannot fly")

class Sparrow(FlyingBird):
    def chirp(self):
        print("Chirping")

class Penguin(NonFlyingBird):
    def swim(self):
        print("Swimming")

CONCLUSION:

Make sure that inheritance is about behavior, not about data. Make sure that the contract of base types is adhered to. Make sure to adhere to the required concepts.