Python 7b – Functions

What is a function?

A function is a subroutine that accepts input values (called parameters), performs a task, and returns a result to the caller. Functions help you avoid repetition, test logic in isolation, and keep programs tidy.

1. Functions take inputs via parameters.
2. Functions return a value using return.

Your first function

This function adds two numbers and returns the result:

def add_numbers(num1, num2):
    # This function adds two numbers and returns the result
    return num1 + num2

result = add_numbers(5, 3)
print("The result is", result)

Example: sphere surface area

The surface area of a sphere is 4 × π × r². This function takes a radius and returns the area:

import math

def sphere_area(radius):
    # Surface area of a sphere = 4 * pi * r^2
    return 4 * math.pi * (radius ** 2)

area = sphere_area(5)
print("The sphere's surface area is", area)

Practice task: volume of a cylinder

Ask the user for a cylinder’s radius and height. Do the calculation in a function (π × r² × h) and return the value. Extension: round to 2 decimal places.

import math

def cylinder_volume(radius, height):
    volume = math.pi * (radius ** 2) * height
    return round(volume, 2)

# Example usage:
r = float(input("Enter the cylinder's radius: "))
h = float(input("Enter the cylinder's height: "))
print("The volume of the cylinder is", cylinder_volume(r, h))

Default & keyword parameters

Provide defaults to make parameters optional, and use keyword arguments for clarity:

def greet(name, punctuation="!"):
    return "Hello, " + name + punctuation

print(greet("Alice"))            # uses default "!"
print(greet(name="Bob", punctuation=" :)"))  # keyword args

Tips & best practices