Python Functions and Automation

After this lesson, you will be able to: Write reusable functions with parameters and return values, and use them to automate repeated tasks.

Functions let you name a chunk of logic and reuse it. They are the single most important tool for keeping code readable as it grows. This lesson covers function definitions, parameters, return values, scope, and how to break a real problem into small functions.

Prerequisites:Python Data Structures

Defining and calling a function

Use def to define a function. Parameters go in parentheses; return sends a value back to the caller.

python

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

message = greet("Alex")
print(message)         # Hello, Alex!
print(greet("World"))   # Hello, World!

Multiple parameters and default values

Parameters can have defaults; named arguments make calls more readable.

python

def calc_total(price, tax_rate=0.08):
    return price + (price * tax_rate)

print(calc_total(20))                 # uses default 8% tax
print(calc_total(20, tax_rate=0.10))  # 10% tax

Importing built-in modules

Python ships with hundreds of useful modules. Import them with import.

python

import random
import datetime

print(random.randint(1, 6))     # roll a die
print(datetime.date.today())     # today's date

Try it: refactor your guessing game

Take last lesson's guessing-game exercise and clean it up using functions.

1
Wrap the game logic in a function called play_round() that returns the number of guesses
2
Write a separate function ask_yes_no(prompt) that returns True/False
3
Use a while loop in main() to play repeatedly until ask_yes_no("Play again?") returns False
4
Print the all-time best score from the leaderboard at the end

Quick Check

What does this function return: def f(x): print(x)?

Trick question, what does Python use when you don't write a return statement?

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