How to Use Global Variable in Python: Syntax and Examples

In Python, to use a global variable inside a function, declare it with the global keyword before modifying it. This tells Python to use the variable defined outside the function instead of creating a new local one.

Syntax

Use the global keyword inside a function to tell Python you want to use the variable defined outside the function. This allows you to modify the global variable instead of creating a new local one.

• global variable_name: Declares the variable as global inside the function.
• Modify or assign the variable after declaring it global.

python

x = 5

def change_global():
    global x
    x = 10

change_global()
print(x)

Output

10

Example

This example shows how to change a global variable inside a function using the global keyword. Without global, the function would create a new local variable instead of changing the global one.

python

counter = 0

def increment():
    global counter
    counter += 1

print("Before:", counter)
increment()
print("After:", counter)

Output

Before: 0
After: 1

Common Pitfalls

One common mistake is to modify a global variable inside a function without declaring it global. This creates a new local variable instead of changing the global one, which can cause bugs.

Another pitfall is to use global unnecessarily when you only need to read the global variable, not modify it.

python

x = 5

def wrong_change():
    x = 10  # This creates a local variable, does not change global x

wrong_change()
print(x)  # Output will be 5, not 10

def correct_change():
    global x
    x = 10  # This changes the global variable

correct_change()
print(x)  # Output will be 10

Output

5
10

Quick Reference

• Use global only when you need to modify a global variable inside a function.
• Reading a global variable inside a function does not require global.
• Be careful to avoid accidental local variables shadowing globals.

Key Takeaways

Declare a variable as global inside a function using the global keyword to modify it.

Without global, assigning to a variable inside a function creates a new local variable.

You do not need global to read a global variable inside a function.

Using global unnecessarily can make code harder to understand.

Always test changes to global variables carefully to avoid bugs.