NameError: name ‘self’ is not defined

In Python, the error message:

NameError: name 'self' is not defined

occurs when you try to use self inside a class method, but Python does not recognize it.

This usually happens in object-oriented programming (OOP) when defining class methods incorrectly.

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1. What Does the Error Mean?

Python uses self as a reference to the instance of a class. It must be explicitly included as the first parameter in instance methods.

If self is missing or incorrectly used, Python does not know what self refers to, leading to this error.

Example of the Error:

class MyClass:
    def say_hello():
        print("Hello, World!")
        print(self)  # Trying to use 'self' without defining it

obj = MyClass()
obj.say_hello()

Error Output:

NameError: name 'self' is not defined

Why?

• The method say_hello() does not have self as its first parameter.
• When calling obj.say_hello(), Python expects self, but it is missing.

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2. Causes and Solutions

Cause 1: Missing self in Instance Methods

When defining a class method, self must be the first parameter.

Incorrect Code:

class Car:
    def start_engine():  # 'self' is missing
        print("Engine started!")

my_car = Car()
my_car.start_engine()  # Error

Error Output:

TypeError: start_engine() takes 0 positional arguments but 1 was given

Solution: Add self as the First Parameter

class Car:
    def start_engine(self):  # Correct: 'self' is included
        print("Engine started!")

my_car = Car()
my_car.start_engine()  # No error

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Cause 2: Using self Outside of a Method

If you use self outside of an instance method, Python does not recognize it.

Incorrect Code:

class Animal:
    print(self)  # Error: 'self' is not defined

dog = Animal()

Error Output:

NameError: name 'self' is not defined

Why?

• self only exists inside instance methods.

Solution: Use self Inside a Method

class Animal:
    def show_self(self):
        print(self)  # Now 'self' is valid

dog = Animal()
dog.show_self()

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Cause 3: Incorrect Use of self in __init__

If self is not passed in __init__(), the instance variables won’t be set.

Incorrect Code:

class Person:
    def __init__(name, age):  # 'self' is missing
        self.name = name  # Error
        self.age = age

p = Person("John", 30)

Error Output:

NameError: name 'self' is not defined

Solution: Add self as the First Parameter

class Person:
    def __init__(self, name, age):  # Corrected
        self.name = name
        self.age = age

p = Person("John", 30)
print(p.name, p.age)  # Output: John 30

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Cause 4: Calling a Method Without an Instance

Trying to call an instance method directly from a class without an instance can cause this error.

Incorrect Code:

class Computer:
    def display_specs(self):
        print("16GB RAM, i7 Processor")

Computer.display_specs()  # Calling without an instance

Error Output:

TypeError: display_specs() missing 1 required positional argument: 'self'

Solution: Create an Instance Before Calling

comp = Computer()
comp.display_specs()  # No error

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Cause 5: Using self in a Static Method

Static methods do not receive self automatically.

Incorrect Code:

class MathOperations:
    @staticmethod
    def add(a, b):
        return self.a + self.b  # 'self' is invalid here

print(MathOperations.add(3, 4))

Error Output:

NameError: name 'self' is not defined

Why?

• Static methods do not receive self because they are independent of class instances.

Solution: Remove self in Static Methods

class MathOperations:
    @staticmethod
    def add(a, b):  # No 'self' here
        return a + b  # Corrected

print(MathOperations.add(3, 4))  # Output: 7

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Cause 6: Using self in a Class Method Without cls

Class methods should use cls, not self.

Incorrect Code:

class Library:
    books = 100

    @classmethod
    def show_books(self):  # Wrong: Use 'cls' instead of 'self'
        print(f"Total books: {self.books}")

Library.show_books()

Error Output:

AttributeError: type object 'Library' has no attribute 'books'

Solution: Use cls Instead of self

class Library:
    books = 100

    @classmethod
    def show_books(cls):  # Corrected
        print(f"Total books: {cls.books}")

Library.show_books()  # Output: Total books: 100

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3. Best Practices to Avoid This Error

To prevent "NameError: name 'self' is not defined", follow these best practices:

Always Include self in Instance Methods

class Example:
    def my_method(self):
        print("Method called")

Use cls for Class Methods

class Example:
    @classmethod
    def class_method(cls):
        print("Class method")

Use self Only Inside Instance Methods

class Example:
    def __init__(self, value):
        self.value = value  # Correct

    def show(self):
        print(self.value)

Use Static Methods When self Is Not Needed

class Example:
    @staticmethod
    def static_method():
        print("Static method")

Create Instances Before Calling Instance Methods

obj = Example()
obj.my_method()  # No error