Abstract Base Classes

Syntax

from abc import ABC, abstractmethod

Examples

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

# shape = Shape()  # would raise TypeError - can't instantiate an abstract class

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    def area(self):
        return 3.14159 * self.radius ** 2

class Square(Shape):
    def __init__(self, side):
        self.side = side

    def area(self):
        return self.side ** 2

shapes = [Circle(5), Square(4)]
for shape in shapes:
    print(f"{type(shape).__name__}: {shape.area():.2f}")

from abc import ABC, abstractmethod

class Employee(ABC):
    def __init__(self, name):
        self.name = name

    @abstractmethod
    def calculate_pay(self):
        pass

    def display(self):  # concrete method, shared by all subclasses
        print(f"{self.name}: ${self.calculate_pay():.2f}")

class SalariedEmployee(Employee):
    def __init__(self, name, salary):
        super().__init__(name)
        self.salary = salary

    def calculate_pay(self):
        return self.salary / 12

emp = SalariedEmployee("Fola", 48000)
emp.display()  # Fola: $4000.00

Best practices

• Use abstract base classes to guarantee that every subclass implements a required set of methods, catching missing implementations early
• Combine abstract methods with regular concrete methods on the same base class for shared functionality plus enforced customization points
• Remember you cannot instantiate a class that still has unimplemented abstract methods - Python raises a TypeError immediately
• Reach for ABCs when designing a plugin-style system or a family of related classes that must all support the same interface

At a glance

Purpose

Scripting and general-purpose applications

File extension

.py

Runs in

Python interpreter

Usually used with

Python standard library and packages

Specifications & further reading

• Python language reference

Related Python documentation