Local and Global Variables

Local Variables

A local variable is defined inside a function and can only be used within that function.

It is created when the function is called and is destroyed once the function finishes execution.

Example:

def my_function():
    y = 5  # Local variable
    print(y)
 
my_function()
print(y)  # This will cause an error because 'y' is not accessible outside the function.
 

Output:

5
NameError: name 'y' is not defined

Global Variables

A global variable is declared outside of all functions and can be accessed from any function within the code.

Example:

x = 10  # Global variable
 
def my_function():
    print(x)  # Accessing the global variable inside the function
 
my_function()
print(x)  # Accessing the global variable outside the function
 

Output:

10
10
 

The global Keyword

The global keyword is used inside a function to indicate that a variable is defined in the global scope, allowing the function to modify its value.

Example:

x = 10  # Global variable
 
def my_function():
    global x
    x = 5  # Modifying the global variable
    print(x)
 
my_function()
print(x)  # The value of 'x' is now changed globally
 

Output:

5
5
 

Global Variables Without global

If you attempt to modify a global variable inside a function without using the global keyword, Python will treat it as a local variable, leading to an error.

Example:

x = 10
 
def my_function():
    x = x + 5  # This will cause an error because 'x' is treated as a local variable
    print(x)
 
my_function()
 

Error:

UnboundLocalError: local variable 'x' referenced before assignment
 

Summary Table

Good Practices

1. Limit the use of global variables: Overusing them can make your code harder to debug and maintain.
2. Prefer passing arguments: Instead of modifying global variables, pass variables as arguments to functions.
3. Encapsulation: Keep variables localized to where they are needed to avoid unintended side effects.