06_functions.md

Functions

Sometimes (often), you will have a bit of code that you want to use in several different places throughout a program, or even in other programs you write later. Writing the exact same code over and over again is not fun for anyone. It's boring, but it's also error-prone and inefficient. Luckily, you can define your own functions and use them instead of copying and pasting over and over again.

Defining a function

A function definition starts with the def keyword, followed by the name of your new function, and any arguments in parentheses. Then you write your code and usually end it with a return statement, which is the output of the function. The return statement lets you assign the value of a function to a variable.

Let's say you wanted to define a function that takes one argument, a number, and adds three to it, then returns that value:

def add_three(num):
    return num+3

Then you could call your new function later on:

add_three(4)

would return 7.

You can also use multiple arguments:

def product_squared(x, y):
    return (x*y)**2

a = 13
b = 22
product_squared(a, b)

Try it yourself: functions

Make a file called functions.py.

Write a function that takes two numbers and returns the sum of their cubes.

Call it using any two numbers you'd like.

More complex functions

One-line functions are almost boring. Sure, they can still save some time, for example:

import datetime
def date():
    return datetime.datetime.now().strftime("%A, %B %d, %Y at %I:%M%p")

print(date())

But normally, you would write a function because you have at least a few lines of code that you want to be able to reference in one short identifier.

def compare(a, b):
    print("Comparing " + str(a) + " to " + str(b) + "...")
    if a > b:
        print(str(a) + " is bigger.")
    elif a < b:
        print(str(b) + " is bigger.")
    else:
        print("They're the same!")
    print("\n")

compare(1, 2)
compare(5*2, 2*5)
compare(555, 1)

You could even take it a step further:

def compare_list(lst):
    for pair in lst:
        compare(pair[0], pair[1])

pairs = [[1, 2], [5*2, 2*5], [555, 1]]
compare_list(pairs)

Function arguments are not limited to numbers. See examples.md for more.

Up next: Object-oriented programming

Head over to 07_object-oriented.md for the last section!