Fix Python NotImplementedError (2025 Guide)

Posted on: March 23, 2025

Encountered a "NotImplementedError" in Python? This error occurs when a method or function that should be implemented is called but hasn’t been. Let’s fix it fast in this 2025 guide!

What Causes "NotImplementedError"?

NotImplementedError is raised when an abstract method or placeholder function is invoked without an implementation. It’s common in object-oriented programming and library usage. Causes include:

• Abstract Base Classes: Calling an unimplemented method from an ABC.
• Placeholder Code: Using a function marked as "not implemented."
• Incomplete Subclassing: Failing to override a required method.

# This triggers "NotImplementedError"
from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def speak(self):
        pass

class Dog(Animal):
    pass  # Forgot to implement speak()

dog = Dog()
dog.speak()  # Error

How to Fix It: 3 Solutions

(Diagram: Developer implements method, resolves error, runs successfully.)

Solution 1: Implement the Method

# Wrong
from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def speak(self):
        pass

class Dog(Animal):
    pass

dog = Dog()
dog.speak()

# Fixed
from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def speak(self):
        pass

class Dog(Animal):
    def speak(self):
        return "Woof!"

dog = Dog()
print(dog.speak())  # "Woof!"

Override the abstract method with a concrete implementation.

Solution 2: Use Try-Except

# Wrong
def feature():
    raise NotImplementedError("This feature isn’t ready yet!")
feature()

# Fixed
def feature():
    raise NotImplementedError("This feature isn’t ready yet!")

try:
    feature()
except NotImplementedError as e:
    print(f"Caught: {e}")

Catch NotImplementedError to handle it gracefully.

Solution 3: Provide a Default Implementation

# Wrong
from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def speak(self):
        pass

class Dog(Animal):
    pass

dog = Dog()
dog.speak()

# Fixed
from abc import ABC, abstractmethod

class Animal(ABC):
    def speak(self):  # Default implementation
        return "No sound"

class Dog(Animal):
    def speak(self):  # Optional override
        return "Woof!"

dog = Dog()
print(dog.speak())  # "Woof!"

Define a default behavior in the base class if overriding isn’t mandatory.

Quick Checklist

• Missing method? (Implement it)
• Expected error? (Use try-except)
• Optional feature? (Add default)

Conclusion

The "NotImplementedError" in Python flags missing implementations, but with these 2025 solutions, you’ll resolve it quickly. Got another Python error? Let us know in the comments!