Functions

What You Will Learn

A function is a reusable block of code that performs a specific task. Instead of writing the same code multiple times, you define it once and call it whenever you need it.

Defining a Function

Use def to define a function:

def greet():
    print("Hello!")
    print("Welcome to Python.")

greet()
greet()

Expected output:

Hello!
Welcome to Python.
Hello!
Welcome to Python.

Define the function once, call it as many times as you want.

Parameters — Giving Information to a Function

Parameters let you pass information into a function:

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

greet("Alice")
greet("Bob")

Expected output:

Hello, Alice!
Hello, Bob!

name is a parameter — a variable that receives the value you pass when calling the function.

Multiple Parameters

def add(a, b):
    print(f"{a} + {b} = {a + b}")

add(3, 4)
add(10, 25)

Expected output:

3 + 4 = 7
10 + 25 = 35

Return Values

A function can give back a result using return:

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

result = add(3, 4)
print(result)
print(add(10, 25))

Expected output:

7
35

The returned value can be stored in a variable or used directly.

Default Parameters

You can give parameters default values:

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

greet("Alice")
greet("Bob", "Hi")
greet("Carol", greeting="Hey")

Expected output:

Hello, Alice!
Hi, Bob!
Hey, Carol!

If you do not pass a value for greeting, it uses the default "Hello".

Keeping Functions Small

Each function should do one thing. Here is a well-structured example:

def celsius_to_fahrenheit(celsius):
    return celsius * 9 / 5 + 32

def fahrenheit_to_celsius(fahrenheit):
    return (fahrenheit - 32) * 5 / 9

def show_conversion(celsius):
    f = celsius_to_fahrenheit(celsius)
    print(f"{celsius}°C = {f:.1f}°F")

show_conversion(0)
show_conversion(100)
show_conversion(37)

Expected output:

0°C = 32.0°F
100°C = 212.0°F
37°C = 98.6°F

What You Learned

• def defines a function
• Parameters pass information into the function
• return sends a result back to the caller
• Default parameters make arguments optional
• Functions should do one thing well

In the next lesson, you will learn about lists — one of Python’s most useful data structures.