How to Fix Python’s NameError: name ‘…’ is not defined

What is NameError: name '...' is not defined?

This error occurs when the Python interpreter encounters a name (variable, function, class) that it doesn’t recognize. Essentially, you are trying to use something that hasn’t been created or assigned a value yet. It is one of the most common errors for beginners.

Common Causes and Solutions

Let’s look at the typical reasons why you might see a NameError and how to resolve them.

1. Misspelling a Variable or Function Name

A simple typo is the most frequent cause. Python is case-sensitive, so myVariable is different from myvariable.

Error Example:

message = "Hello, World!"
print(mesage)

Output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'mesage' is not defined

Solution: Correct the typo. Ensure the name used matches the name at declaration.

message = "Hello, World!"
print(message) # Corrected from 'mesage'

2. Using a Variable Before Assignment

You must assign a value to a variable before you can use it.

Error Example:

if some_condition:
    user_name = "Alice"

print(user_name) # NameError if some_condition is False

Solution: Initialize the variable with a default value before the conditional block.

user_name = "Guest" # Initialize with a default value
if some_condition:
    user_name = "Alice"

print(user_name)

3. Variable Scope Issues

A variable defined inside a function (a local variable) cannot be accessed from outside that function.

Error Example:

def greet():
    greeting = "Hello from inside the function!"
    print(greeting)

greet()
print(greeting) # This will cause a NameError

Output:

Hello from inside the function!
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'greeting' is not defined

Solution: If you need to use the value outside, return it from the function and assign it to a new variable.

def greet():
    greeting = "Hello from inside the function!"
    return greeting

returned_greeting = greet()
print(returned_greeting)

4. Forgetting to Import a Module or Name

When using modules from the standard library or third-party packages, you must import them first.

Error Example:

# Forgetting to import the 'math' module
print(math.sqrt(25))

Output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'math' is not defined

Solution: Add the required import statement at the top of your script.

import math

print(math.sqrt(25))

By checking for these common mistakes, you can quickly identify and fix most NameError exceptions in your Python code.