3.11 Functions in Python

What is a Function?

A function is a named block of reusable code that performs a specific task. Instead of writing the same code repeatedly, you define it once in a function and call it whenever needed.

Functions help in:

• Code reusability — write once, use many times
• Problem decomposition — break a large problem into smaller sub-problems
• Readability — makes programs easier to understand
• Maintainability — easier to fix or update code in one place

---

Types of Functions in Python

---

Defining a Function

Use the def keyword followed by the function name and parentheses:

def function_name(parameters):
    # body of the function
    statement(s)

Example:

def greet():
    print("Hello, World!")

greet()   # calling the function

Output:

Hello, World!

---

Function Parameters and Arguments

• Parameter — a variable listed inside the parentheses in the function definition.
• Argument — the actual value passed to the function when it is called.

def greet(name):        # 'name' is a parameter
    print("Hello,", name)

greet("Ali")            # 'Ali' is an argument
greet("Sara")

Output:

Hello, Ali
Hello, Sara

Multiple Parameters

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

add(3, 5)   # Output: 8

---

The return Statement

A function can return a value to the caller using the return statement.

def square(n):
    return n * n

result = square(4)
print(result)   # Output: 16

• Once return is executed, the function ends.
• A function without a return statement returns None by default.

---

Default Parameters

You can assign a default value to a parameter. If no argument is passed, the default is used.

def greet(name="Guest"):
    print("Hello,", name)

greet()          # Output: Hello, Guest
greet("Ahmed")   # Output: Hello, Ahmed

---

Scope of Variables

• Local variable — defined inside a function; accessible only within that function.
• Global variable — defined outside all functions; accessible throughout the program.

x = 10          # global variable

def show():
    y = 5       # local variable
    print(x)    # can access global x
    print(y)

show()
# print(y)    # Error! y is not accessible here

---

Problem Decomposition Using Functions

Functions allow you to decompose (break down) a complex problem into smaller, manageable sub-problems.

Example — Calculate area and perimeter of a rectangle:

def area(length, width):
    return length * width

def perimeter(length, width):
    return 2 * (length + width)

l = 5
w = 3
print("Area:", area(l, w))           # Output: Area: 15
print("Perimeter:", perimeter(l, w)) # Output: Perimeter: 16

Each sub-problem (area, perimeter) is solved independently in its own function.

---

Summary