Your own functions

What you'll learn

Write your own functions with def
Pass values in with parameters and give them back with return
Reuse functions inside other functions

Why your own functions?

You’ve been using functions all along: print(), len(), input(). Now you build your own! A function is a mini program with a name. You write it once and can then call it as often as you like.

def greet():
    print("Hello!")
    print("Great to have you here.")

greet()
greet()

def means “define”. The code inside the function is indented and only runs when you call the function.

Parameters: passing values in

Inside the brackets you write parameters, placeholders for values you pass in when calling:

def greet(name, language="en"):
    if language == "de":
        print(f"Hallo {name}!")
    else:
        print(f"Hello {name}!")

greet("Mia")
greet("Tom", "de")

language="en" is a default value: if you leave it out when calling, "en" is used.

return: giving results back

print() only shows something. With return the function gives back a result that you can keep calculating with:

def area(width, height):
    result = width * height
    return result

room = area(4, 3)
hall = area(5, 1)
print(f"Together: {room + hall} m²")

Press Steps: when Python jumps into the function you see a separate memory area “Inside function area()”. Variables like result only exist there – after return they’re gone again.

Remember: After return the function ends immediately. Code after it inside the function doesn’t run any more.

Combining functions

Functions get really powerful when they use each other. That keeps every function short and easy to understand:

def in_euros(cents):
    return cents / 100

def price_text(cents):
    return f"{in_euros(cents):.2f} €"

print(price_text(250))
print(price_text(1999))

:.2f in the f-string means: show exactly 2 decimal places.

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

def double(number):
    return number * 2


print(double(3) + double(1))

Your task

Your turn

Write two functions. is_even(number) returns True if the number is even, otherwise False (tip: number % 2 is the remainder when dividing by 2). count_even(numbers) gets a list of numbers and returns how many of them are even. Use is_even for that.

def is_even(number):
    # return True or False
    pass


def count_even(numbers):
    # count with the help of is_even
    pass


print(is_even(4))
print(count_even([1, 2, 3, 4, 6]))

Something to think about
A number is even when number % 2 == 0. Give the result back with return.
Which command helps?
Start with count = 0, go through the list and increase count whenever is_even(number) is True. At the end return count.
Call is_even inside count_even instead of repeating the calculation.

Gap code – fill in the ___

                        def is_even(number):
    return ___


def count_even(numbers):
    count = ___
    for number in ___:
        if ___:
            count = ___
    return ___


print(is_even(4))
print(count_even([1, 2, 3, 4, 6]))

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

def is_even(number):
    return number % 2 == 0


def count_even(numbers):
    count = 0
    for number in numbers:
        if is_even(number):
            count = count + 1
    return count


print(is_even(4))
print(count_even([1, 2, 3, 4, 6]))

For fast learners

Pro task

Write is_palindrome(text). A palindrome reads the same forwards and backwards, like “Anna” or “racecar”. Upper and lower case and spaces don't count: “Never odd or even” is a palindrome too. The function returns True or False.

def is_palindrome(text):
    return False


print(is_palindrome("Anna"))
print(is_palindrome("Python"))

Show pro solution

def is_palindrome(text):
    clean = text.lower().replace(" ", "")
    return clean == clean[::-1]


print(is_palindrome("Anna"))
print(is_palindrome("Python"))

Practise more in the training roomEndless new tasks on this topic – with XP and streaks.