Variable Scope

Where does a variable live?

In Python, not all variables are accessible from everywhere in your code. The region where a variable is defined and can be accessed is called its Scope.

Local Scope

A variable created inside a function belongs to the local scope of that function. It only exists while the function is running.

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def my_function():
    x = 10  # Local variable
    print(x)

my_function()
# print(x) # ERROR! x is not defined outside the function

Global Scope

A variable created in the main body of a Python file is a global variable and belongs to the global scope. Global variables are available from within any scope, next to global or local.

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x = 300 # Global variable

def my_function():
    print(x) # Accessing global variable

my_function()
print(x)

Naming Conflicts

If you operate with the same variable name inside and outside of a function, Python will treat them as two separate variables: one local and one global.

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x = 50

def my_function():
    x = 20 # This creates a NEW local variable x
    print(f"Local x: {x}")

my_function()
print(f"Global x: {x}")

The global Keyword

If you need to modify a global variable from inside a function, you must use the global keyword.

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count = 0

def increment():
    global count
    count += 1
    print(f"Internal count: {count}")

increment()
print(f"External count: {count}")

Interactive Practice

Think about a video game. The player_score might be global, but a bonus_points variable inside a “calculate_level_end” function should probably be local.