Functions: Reusable Superpowers 🦸

You've already been using functions like len(...) and print(...) this whole time — those are built into Python. Now let's learn to write your own.

def greet(name):
    return "Hello, " + name + "!"

message = greet("Sam")
print(message)  # Hello, Sam!

def starts a new function, followed by a name and, in parentheses, any inputs it needs (called parameters). Whatever comes after return gets sent back to wherever the function was called — like an answer being handed back.

Default parameter values

You can give a parameter a default value, so callers can leave it out if they want:

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

greet("Sam")          # "Hello, Sam!"
greet("Sam", "Hi")    # "Hi, Sam!"

Calling functions from other functions

Functions can call other functions — this is called composition, and it's a great way to reuse code instead of repeating yourself:

def double(n):
    return n * 2

def quadruple(n):
    return double(double(n))

Functions that answer yes/no questions

Functions that return True or False are often named starting with is_ — they're called predicates, and they're great building blocks for if statements elsewhere in your code.

A sneak peek at recursion

A function can even call itself! This is called recursion. It always needs a base case that stops it, or it would call itself forever:

def countdown_sum(n):
    if n <= 0:
        return 0
    else:
        return n + countdown_sum(n - 1)

Let's put all of this into practice!