First Functions

Create a file called my_functions.py and take a look at the code below:

def add_five(number):
    return number + 5

what_is_this_number = add_five(10) # It's a hard question but I guess 15...
print(what_is_this_number)

We're actually doing two things here: defining the add_5 function and then calling it. We define the add_five function by first typing def, then the function name "add_five", we then have brackets which contain parameters and finally on that line a colon. The code of the function simply consists of the return keyword and a maths expresision that adds 5 to number.

That's a lot to take in, but it's simple. The parameters are simply inputs for the function. The colon says it's a new block of code i.e. all the code that's indented belongs to the function. The return keyword simply says what the function outputs. Let's see some other functions:

# This function takes a string called 'message' and returns a new string
# that adds the word adios to the message
def say_goodbye(message):
    return message + ' ¡Adios!'

print(say_goodbye('Hi my name is Paul!'))

# Functions you defined can call other functions as well
def add_ten(number):
    with_five_more = add_five(number)
    return add_five(with_five_more)

print(add_ten(10)) # You'll get 20

# We can use the output of one function as input for another
def add_ten_version2(number):
    return add_five(add_five(number))

print(add_ten_version2(10)) # You'll get 20

So far our functions have been pretty much the same format: def keyword, function name, a parameter in brackets, some code that returns a value at the end. We got some flexibility, especially with the parameters and return values:

# Yep, you can have multiple parameters
def my_multiply(x, y):
    return x * y

print(my_multiply(7, 5)) # 35

# Sometimes you don't need parameters
def greetings():
    return '¡Hola!'

print(greetings()) # You must still use the brackets even without parameters!

# Some functions may not return values
def haiku():
    print('Do I want to fight? Chuck Norris roundhouse kicked me! I will fight no more...')

haiku() # Because this function prints we don't call the print statement again

There's something to note about the return statement, it ends the execution of the function. So every line of code that comes after it is never run. Try this quick example:

def maths_for_fun(num1, num2, num3):
    mult_nums = num1 * num3
    div_nums = num2 / num1
    return mult_nums - div_nums
    # This code with never run
    add_nums = num2 + num3
    return add_nums

print(maths_for_fun(5,25,6)) # 25 instead of 31