Functions Introduction

An introduction to the use of Functions in Python

Functions Introduction

A function is a block of instructions that performs an action and, once defined, can be reused. Functions make code more modular, allowing you to use the same code over and over again.

Python has a number of built-in functions that you may be familiar with, including:

  • print() which will print an object to the terminal
  • int() which will convert a string or number data type to an integer data type
  • len() which returns the length of an object
Function names include parentheses and may include parameters.

In this tutorial, we’ll go over how to define your own functions to use in your coding projects.


Let’s start with turning the classic “Hello, World!” program into a function.

A function is defined by using the def keyword, followed by a name of your choosing, followed by a set of parentheses which hold any parameters the function will take (they can be empty), and ending with a colon.

In this case, we’ll define a function named hello():

def hello():

This sets up the initial statement for creating a function.

From here, we’ll add a second line with a 4-space indent to provide the instructions for what the function does. In this case, we’ll be printing Hello, World! to the console:

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

Our function is now fully defined, but if we run the program at this point, nothing will happen since we didn’t call the function.

So, outside of our defined function block, let’s call the function with hello():

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


Now, let’s run the program:

You should receive the following output:

Hello, World!

Functions can be more complicated than the hello() function we defined above. For example, we can use for loops, conditional statements, and more within our function block.

For example, the function defined below utilizes a conditional statement to check if the input for the name variable contains a vowel, then uses a for loop to iterate over the letters in the name string.

# Define function names()
def names():
    # Set up name variable with input
    name = str(input('Enter your name: '))
    # Check whether name has a vowel
    if set('aeiou').intersection(name.lower()):
        print('Your name contains a vowel.')
        print('Your name does not contain a vowel.')

    # Iterate over name
    for letter in name:

# Call the function

The names() function we defined above sets up a conditional statement and a for loop, showing how code can be organized within a function definition. However, depending on what we intend with our program and how we want to set up our code, we may want to define the conditional statement and the for loop as two separate functions.

Defining functions within a program makes our code modular and reusable so that we can call the same functions without rewriting them.

Defining a Function

Write a program that creates a single function that adds two numbers together and outputs the total.

Write a program that creates a two functions, one that adds two numbers together and outputs the total. The other should give the length of a string.

Write a program that creates two functions. Each function should use variables to store data and then carry out a process of your choice.

11.1 Functions Introduction Task
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