Defining Functions

A function is a named, reusable block of code:

def greet(name):
    print(f"Hello, {name}!")

greet("Minh")     # Hello, Minh!
greet("Lan")      # Hello, Lan!

Anatomy: def starts the definition; the name follows the same rules as variables; parameters (name) are inputs the function receives; the body is indented under the def line. Defining does not run the code — calling does.

Why functions exist

• Reuse: write once, call anywhere.
• Naming: send_report() documents intent better than 30 loose lines.
• Testing: a function is a unit you can check in isolation.
• Decomposition: big problems become small named steps.

return: produce a result

print shows a human something; return hands a value back to the program:

def add(a, b):
    return a + b

total = add(2, 3)     # total is now 5
print(add(10, 5) * 2) # return values compose into expressions

return also exits the function immediately. A function with no return statement returns None.