Python Local vs Global Variables: Scope, Usage & Best Practices

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What Is a Variable in Python

A variable is a name that references an object in your computer's memory. In Python, variables don't have fixed types — the type belongs to the object the variable points to.

x = 42
x = "now a string"

Local Variables in Python

Defining Local Variables

A local variable is defined inside a function and is only accessible within that function. Once the function finishes executing, the local variable is removed from memory.

def say_hello():
    name = "Python"  # local variable
    print("Hello,", name)

say_hello()
# print(name)  # Error: name is not defined

The variable name only exists inside the say_hello function and cannot be accessed outside it.

Benefits of Local Variables

Local variables provide data isolation and prevent accidental changes from other parts of your program. They also automatically free up memory after the function finishes.

Global Variables in Python

What Are Global Variables

A global variable is declared outside all functions and is accessible anywhere in the program file. These variables retain their values throughout the entire program execution.

language = "Python"  # global variable

def show():
    print("Language:", language)

show()  # Language: Python

Problems with Modifying Global Variables

If you try to modify a global variable inside a function without declaring it, you'll get an error:

counter = 0

def increment():
    counter += 1  # UnboundLocalError!

Python treats counter as a local variable because of the assignment operation, but it hasn't been defined locally.

The `global` Keyword

Using `global` to Modify Global Variables

The global keyword lets you modify global variables from inside a function:

count = 0

def increment():
    global count
    count += 1

increment()
print(count)  # 1

Using `global` in Mathematical Operations

total = 0

def add(x):
    global total
    total += x

add(10)
add(20)
print(total)  # 30

When to Use `global`

Use the global keyword sparingly — it can make debugging harder and your code less readable.

Variable Scope: The LEGB Rule

How Python Searches for Variables

Python looks up variables using the LEGB rule in this order:

L (Local) — inside the current function
E (Enclosing) — in nested functions
G (Global) — at the module level
B (Built-in) — Python's built-in names

LEGB Rule Example

x = "global"

def outer():
    x = "enclosing"
    
    def inner():
        x = "local"
        print(x)
    
    inner()

outer()  # local

Python finds x in the local scope and uses that value.

The `nonlocal` Keyword

Accessing Variables in Nested Functions

The nonlocal keyword lets you work with variables from an outer (but non-global) scope:

def outer():
    count = 0
    
    def inner():
        nonlocal count
        count += 1
        return count
    
    return inner

f = outer()
print(f())  # 1
print(f())  # 2