What are global, local, and nonlocal scopes in Python

Understand scopes and their usages with examples.

Scope is defined as an area where eligible variables can be accessed. To enforce security, programming languages provide means by which a user can explicitly define these scopes.

#more

It is important to understand the use of scopes and how to deal with them. In this article, we will see what are the scopes available in Python and how to work with them.

1. Global scope¶

Any variable defined outside a non-nested function is called a global. As the name suggests, global variables can be accessed anywhere.

Example:¶

side = 5 # defined in global scope

def area():
    return side * side

def circumference():
    return 4 * side

print(f"Area of square is {area()}")
print(f"Circumference of square is {circumference()}")

Output:

Area of square is 25
Circumference of square is 20

When a function tries to manipulate global variables, an UnboundLocalError is raised. To overcome this the global variable is redefined inside the function using global keyword. In this way a user can modify global variables without errors.

Example:¶

Without global keyword

side = 5

def multiply_side(factor):
  side *= factor

multiply_side(7)
print(f"Side length is {side}")

Output:

UnboundLocalError: local variable 'side' referenced before assignment

With global keyword

side = 5

def multiply_side(factor):
    global side
    side *= factor

multiply_side(7)
print(f"Side length is {side}")

Output:

Side length is 35

2. Local scope¶

By default, variables defined inside a function have local scope. It implies that local scope variables can be accessed only inside the parent function and nowhere else.

Local variables are destroyed as soon as the scope ceases to exist.

Example:¶

side = 5

def area():
    square_area = side * side # local scope

print(square_area)

Output:

NameError: name 'square_area' is not defined

3. Nonlocal scope¶

Nested functions introduce a new type of scope called as nonlocal scope. When a nested function wants to share the local scope of parent functions, nonlocal keyword is used.

In such cases, declaring parent function variables as global does not work.

Example:¶

Without using nonlocal keyword

side = 5

def half_area():
    area = side * side
    def divide():
        area /= 2

    divide()
    return area

print(half_area())

Output:

UnboundLocalError: local variable 'area' referenced before assignment

Using nonlocal keyword:

side = 5

def half_area():
    area = side * side
    def divide():
        nonlocal area
        area /= 2

    divide()
    return area

print(half_area())

Output:

12.5