ISP.md

4. Interface Segregation Principle

The interface segregation principle (ISP) states that no code should be forced to depend on methods it does not use. Source.

Think of a coffee shop that offers various services, such as making coffee, selling baked goods, and providing free Wi-Fi. The Interface Segregation Principle (ISP) is like saying that instead of having a single line where a customer has to wait to order coffee, buy a croissant, and ask for the Wi-Fi password, it would be better to segregate these interfaces. So, you'd have one line for ordering coffee, another for buying baked goods, and a sign with the Wi-Fi details. This way, customers who just want a croissant aren't stuck behind someone trying to decide which coffee to order, and those who just want the Wi-Fi don't have to queue at all. Each service is an 'interface', and customers only use the ones they need.

If we add the firmware_update function to our Hardware class, it would mean that, all our child classes would need to have the firmware_update function.

class Hardware(ABC):
    
    @abstractmethod
    def functionality():
        pass
    
    @abstractmethod
    def firmware_update():
        pass

Now, this firmware_update function will not be used by the mouse and keyboard usually. So, why do these classes even need to have this function? This violates the ISP. To avoid this, we can have the following update

from abc import ABC, abstractmethod

class Hardware(ABC):
    
    @abstractmethod
    def functionality():
        pass
    
class Firmware(ABC):
    
    @abstractmethod
    def update():
        pass

class Monitor(Hardware, Firmware):
    
    def functionality(self):
        print('To display information on the screen.')
        
    def update():
        print('Updating firmware.')

class Mouse(Hardware):
    
    def functionality(self):
        print('To move cursor on the screen and click on elements.')

class Keyboard(Hardware):
    
    def functionality(self):
        print('To help type the required information.')
        
class Printer(Hardware, Firmware):
    
    def functionality(self):
        print('To print information on the paper.')
        
    def update(self):
        print('Updating firmware.')