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The Interface Segregation Principle

“Many client-specific interfaces are better than one general-purpose interface”
In the contest of classes, an interface is considered, all the methods and properties “exposed”, thus, everything that a user can interact with that belongs to that class.
In this sense, the The Interface Segregation Principle tell us that a class should only have the interface needed (the-single-responsibility-principle) and avoid methods that won’t work or that have no reason to be part of that class.
This problem arises, primarily, when, a subclass inherits methods from a base class that it does not need.

import numpy as np
from abc import ABC, abstractmethod

class Mammals(ABC):
    @abstractmethod
    def swim() -> bool:
        print("Can Swim") 

    @abstractmethod
    def walk() -> bool:
        print("Can Walk") 

class Human(Mammals):
    def swim():
        return print("Humans can swim") 

    def walk():
        return print("Humans can walk") 

class Whale(Mammals):
    def swim():
        return print("Whales can swim") 
	
# ========================================= #
Human.swim()
Human.walk()

Whale.swim()
Whale.walk()

# Humans can swim
# Humans can walk
# Whales can swim
# Can Walk

The sub-class whale can still invoke the method “walk” but it shouldn’t, and we must avoid it.

The way suggested by ISP is to create more client-specific interfaces rather than one general-purpose interface. So, our code example becomes:

from abc import ABC, abstractmethod

class Walker(ABC):
  @abstractmethod
  def walk() -> bool:
    return print("Can Walk") 

class Swimmer(ABC):
  @abstractmethod
  def swim() -> bool:
    return print("Can Swim") 

class Human(Walker, Swimmer):
  def walk():
    return print("Humans can walk") 
  def swim():
    return print("Humans can swim") 

class Whale(Swimmer):
  def swim():
    return print("Whales can swim") 

if __name__ == "__main__":
  Human.walk()
  Human.swim()

  Whale.swim()
  Whale.walk()

# Humans can walk
# Humans can swim
# Whales can swim
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