NameError: global name ‘x’ is not defined

In Python, the error message:

NameError: global name 'x' is not defined

occurs when a variable is referenced globally, but Python does not recognize it because it has not been properly defined or declared.

To understand and fix this error, let’s go through step-by-step explanations, common causes, and solutions.

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1. What Does the Error Mean?

This error specifically occurs when:

• A variable is accessed as a global variable, but it hasn’t been defined in the global scope.
• The global keyword is used incorrectly.
• There is a scoping issue inside functions.

Example of the Error:

def my_function():
    print(x)  # Trying to access 'x' before defining it

my_function()

Error Output:

NameError: global name 'x' is not defined

Here, Python does not recognize x because it was never assigned a value before being accessed.

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2. Causes and Solutions

Let’s explore the different causes of this error and how to fix it.

Cause 1: Using a Global Variable Before Defining It

If you try to access a global variable before defining it, Python will throw a NameError.

Example:

print(x)  # 'x' is used before being defined
x = 10

Error Output:

NameError: global name 'x' is not defined

Solution:

Define the variable before using it.

x = 10
print(x)  # Now, it works fine

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Cause 2: Incorrect Use of the global Keyword in a Function

If you try to modify a global variable inside a function without declaring it using global, Python treats it as a local variable.

Example:

x = 10

def my_function():
    x += 5  # Python assumes 'x' is local but it is not defined

my_function()

Error Output:

UnboundLocalError: local variable 'x' referenced before assignment

Solution:

Use the global keyword to explicitly modify the global variable.

x = 10

def my_function():
    global x
    x += 5  # Now, 'x' is correctly modified

my_function()
print(x)  # Output: 15

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Cause 3: Variable Defined Inside a Function Cannot Be Accessed Globally

A variable declared inside a function is local to that function and cannot be accessed outside.

Example:

def my_function():
    y = 20  # 'y' is local to my_function

print(y)  # Trying to access 'y' outside the function

Error Output:

NameError: global name 'y' is not defined

Solution:

Define the variable outside the function to make it global.

y = 20  # Now 'y' is global

def my_function():
    print(y)  # Works fine

my_function()
print(y)  # No error

Alternatively, return the variable from the function:

def my_function():
    y = 20
    return y

result = my_function()
print(result)  # Works fine

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Cause 4: Forgetting to Declare a Global Variable Before Using It

If you try to use a variable inside a function and declare it as global after using it, Python will raise an error.

Example:

def my_function():
    print(x)  # Trying to access 'x' before defining it
    global x
    x = 10

my_function()

Error Output:

NameError: global name 'x' is not defined

Solution:

Define the variable first, then use it.

x = 10  # Define globally before use

def my_function():
    global x
    print(x)  # Works fine

my_function()

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Cause 5: Forgetting to Import a Module

If you use a function from a module without importing the module, Python will raise a NameError.

Example:

print(math.pi)  # 'math' module is not imported

Error Output:

NameError: global name 'math' is not defined

Solution:

Import the module before using it.

import math
print(math.pi)  # Works fine

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3. Best Practices to Avoid This Error

To prevent NameError: global name 'x' is not defined, follow these best practices:

Define Variables Before Using Them

Always define a variable before using it.

x = 5
print(x)  # No error

Use global Keyword Correctly

When modifying a global variable inside a function, use global properly.

x = 100

def modify_x():
    global x
    x += 50  # Modifies the global 'x'

modify_x()
print(x)  # Output: 150

Ensure Variables Are in the Correct Scope

If a variable is needed outside a function, define it globally or return it.

def set_value():
    y = 30
    return y

result = set_value()
print(result)  # Works fine

Always Import Required Modules

Ensure modules are imported before use.

import random
print(random.randint(1, 10))  # No error

Handle Errors Gracefully

Use a try-except block to handle missing variables.

try:
    print(undeclared_var)
except NameError:
    print("Variable is not defined!")