NotImplementedError: Abstract Method

Why This Happens

NotImplementedError is raised by abstract methods that subclasses must override. Use the abc module for formal abstract base classes that catch missing implementations at instantiation.

The Problem

class Shape:
    def area(self): raise NotImplementedError

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

Circle(5).area()

The Fix

class Shape:
    def area(self): raise NotImplementedError

class Circle(Shape):
    def __init__(self, r): self.r = r
    def area(self):
        import math
        return math.pi * self.r ** 2

Step-by-Step Fix

1. 1

   Implement the method

   Override the abstract method in your subclass.

2. 2

   Check all abstract methods

   Look at the base class for all NotImplementedError methods.

3. 3

   Use ABC

   Use abc.ABC and @abc.abstractmethod for compile-time checks.