27. Understanding the global Keyword

Quiz: 0/5

The global keyword in Python is used to declare that a variable inside a function refers to a variable defined in the global scope. This allows you to modify global variables from within a function. Without the global keyword, assigning a value to a variable inside a function will create a local variable, leaving the global variable unchanged.

Defining global variables

A global variable is defined outside of any function, making it accessible throughout the entire script. Functions can read global variables without any special declaration.

# Global variable
x = 10

def print_x():
    print(x)

print_x()  # Output: 10

In this example, x is a global variable, and the print_x() function accesses it directly without modification.

Modifying global variables with global

To modify a global variable inside a function, you must use the global keyword. Without it, Python will create a local variable instead of changing the global one.

# Global variable
counter = 0

def increment():
    global counter  # Declare that we want to use the global variable
    counter += 1    # Increment the global variable

increment()
print(counter)  # Output: 1

increment()
print(counter)  # Output: 2

Key Points in the Example:

• The increment() function declares counter as global to indicate that changes should apply to the global variable.
• Each call to increment() modifies the global counter variable, allowing changes to persist outside the function.

What happens without the global keyword?

If you attempt to assign a value to a variable inside a function without declaring it as global, Python will create a new local variable. The global variable will remain unchanged.

# Global variable
value = 5

def change_value():
    value = 10  # This creates a new local variable named 'value'
    print(value)

change_value()  # Output: 10
print(value)    # Output: 5 (the global variable remains unchanged)

Key points in the example:

• The change_value() function creates a local variable value that temporarily shadows the global variable.
• The global variable value remains unaffected when you print it after calling change_value().

Key takeaways

1. Using global: The global keyword allows functions to modify a global variable.
2. Local vs. Global Scope: Without global, assigning to a variable inside a function creates a local variable that does not affect the global variable.
3. Best Practices:
   • Minimize the use of global variables to avoid unintended side effects.
   • Consider using function parameters and return values to pass data between functions.