Python Lesson #5: Functions

If variables are the nouns of programming, functions are the verbs. In this lesson we’ll learn a few important functions and how to create our own custom ones!


“Returning” means telling the computer internally. It’s like print() except instead of showing us, it shows the computer. return itself is a keyword we will be using in our functions to make the computer output the result of the function.

When a function returns, it exits, and if there is any code after the return statement, it will not be executed.


The function len() returns the number of items in a sequence (strings, lists, tuples, sets) or mapping (dictionary, which will be covered later).

list = ["item_1", "item_2", "item_3"]


The function input() allows us to take in user input through the terminal. Whatever is typed is returned as a string.

favorite_color = input("What's your favourite colour? ")

This prints out the string you input.

Datatype conversion

type() returns what datatype a certain value is.


To convert a value from one type to another, these functions are needed. These are just a few common ones:

Sometimes, you may want to work with certain values as certain types. For example:

print("5" + "5")
print(5 + 5)

The first prints "55" and the second prints 10.

Creating functions

To create our own functions, use the def keyword:

def function_name(arguments):
    # Actions

Arguments are what we pass into the function, separated by commas. Imagine them like ingredients: Python needs to know what they are before it starts cooking with them. Not all functions require arguments.

Here is a simple function that adds two numbers together:

def add_two_numbers(num_1, num_2):
    return num_1 + num_2

Since return is used, Python stores the value internally. To see it ourselves, we need to print the function call.

print(add_two_numbers(1, 2)) # Prints `3`

We can call functions within our functions, for example adding user input:

def add_two_numbers_with_input():
    x = int(input("Enter your first number: "))
    y = int(input("Enter your second number: "))
    return x + y
print(add_two_numbers_with_input()) # Prints sum of input

Since input() returns a string, we need int() to change it into an integer.

To use this function, we need to call it. Since it adds together user input and not hard coded numbers, we do not need any arguments.

Try it!

Create a function that takes a number as an argument that returns "The number is above 10" if it is greater than 10 and "The number is 10 or below" if it is not.

Then, call the function with a number taken from input() and print the result.


def my_function(number):
    if number > 10:
        return "The number is above 10"
    return "The number is 10 or below"
number = input("Enter a number: ")
result = my_function(number)