Interface Segregation Principle

Clients should not be forced to depend on interfaces they do not use.

This means a class should not be required to implement methods it doesn’t need. ISP encourages splitting large interfaces into more specific ones.

To demonstrate the Interface Segregation Principle (ISP), consider the following example with a Worker interface:

❌ Before Applying ISP

Copy
# worker.py

class Worker:
    def work(self):
        pass

    def eat(self):
        pass

class HumanWorker(Worker):
    def work(self):
        print("Working...")

    def eat(self):
        print("Eating lunch...")

class RobotWorker(Worker):
    def work(self):
        print("Working...")

    def eat(self):
        raise NotImplementedError("Robots don't eat")
# worker.py

class Worker:
    def work(self):
        pass

    def eat(self):
        pass

class HumanWorker(Worker):
    def work(self):
        print("Working...")

    def eat(self):
        print("Eating lunch...")

class RobotWorker(Worker):
    def work(self):
        print("Working...")

    def eat(self):
        raise NotImplementedError("Robots don't eat")

🔴 Problem: RobotWorker is forced to implement the eat method even though it doesn’t make sense for robots.

✅ After Applying ISP

Copy
# interfaces.py

class Workable:
    def work(self):
        pass

class Eatable:
    def eat(self):
        pass
# interfaces.py

class Workable:
    def work(self):
        pass

class Eatable:
    def eat(self):
        pass

Copy
# workers.py

class HumanWorker(Workable, Eatable):
    def work(self):
        print("Working...")

    def eat(self):
        print("Eating lunch...")

class RobotWorker(Workable):
    def work(self):
        print("Working...")
# workers.py

class HumanWorker(Workable, Eatable):
    def work(self):
        print("Working...")

    def eat(self):
        print("Eating lunch...")

class RobotWorker(Workable):
    def work(self):
        print("Working...")

Now:

• HumanWorker implements both Workable and Eatable.
• RobotWorker only implements Workable, not forced to define irrelevant behavior.

This design makes the code:

• ✅ Cleaner and more meaningful
• ✅ Easier to maintain and extend
• ✅ More aligned with real-world roles