Upgrading from Wizard to Inventor 🔧
So far, you have been using spells (commands) that other people wrote:
print()input()range()len()
But what if you want to make your own spell?
What if you want a command called greet_player() or calculate_score()?
You Can! Functions Let You Invent New Commands! ⚡
Before: After:
┌──────────┐ ┌──────────────┐
│ print() │ │ print() │
│ input() │ │ input() │
│ len() │ + │ len() │
└──────────┘ │ greet() │← YOUR spell!
│ add_score() │← YOUR spell!
└──────────────┘
Defining a Function 📝
We use the keyword def (short for “define”).
def greet_player():
print("Welcome, brave adventurer!")
print("Prepare for glory!")
Run this. … Nothing happened? 🤔
That’s because we only defined (created) the spell. We haven’t called (used) it yet!
Calling the Function 🎯
To use it, we just type its name with ():
greet_player()
Output:
Welcome, brave adventurer!
Prepare for glory!
You can call it as many times as you want!
greet_player()
greet_player()
greet_player()
This will print the message 3 times! It’s like pressing a button that does a whole sequence of actions!
🚀 Try This NOW!
What will this code do?
def say_hello():
print("Hello!")
print("How are you?")
say_hello()
say_hello()
Click to check!
It will print: ``` Hello! How are you? Hello! How are you? ``` The function runs twice!🎮 Real-World Connection
Every button in a game is a function! When you click “Jump” in Fortnite, it calls a function jump() that makes your character jump. When you click “Like” on Instagram, it calls a function like_post(). Functions are how programmers organize their code!
Functions with Ingredients (Parameters) 🥄
Some spells need ingredients. print() needs text. Your function can take ingredients too!
def greet(name):
print("Hello", name)
print("Nice to meet you.")
Now we pass the name when we call it:
greet("Elena")
greet("Sam")
greet("Alex")
Output:
Hello Elena
Nice to meet you.
Hello Sam
Nice to meet you.
Hello Alex
Nice to meet you.
Multiple Parameters
You can have MORE than one ingredient!
def introduce(name, age):
print("Hi, I'm", name)
print("I am", age, "years old")
introduce("Sam", 10)
introduce("Alex", 12)
Functions That Give Back Answers (Return) 🎁
So far our functions just print things. But what if we want them to calculate something and give us the answer?
def double_number(number):
return number * 2
result = double_number(5)
print(result) # 10
The return keyword sends the answer back!
Visual Flow:
┌─────────────────────┐
│ double_number(5) │
│ ↓ │
│ 5 * 2 = 10 │
│ ↓ │
│ return 10 │
│ ↓ │
│ result = 10 │
└─────────────────────┘
Return vs. Print
Print shows something on screen. Return gives a value back that you can save or use.
def add(a, b):
return a + b
def greet():
print("Hello!")
# Return example:
total = add(5, 3) # total = 8 (we can save it!)
print(total * 2) # 16
# Print example:
greeting = greet() # greeting = None (nothing to save!)
Why Use Functions? 🤔
1. Laziness (The Good Kind!)
Write code once, use it 100 times.
def calculate_score(points, bonus):
return points * 10 + bonus
level1_score = calculate_score(5, 100)
level2_score = calculate_score(8, 200)
level3_score = calculate_score(12, 500)
2. Organization
Groups related code together (like chapters in a book).
def start_game():
print("Game starting...")
def play_level():
print("Playing...")
def end_game():
print("Game over!")
start_game()
play_level()
end_game()
3. Clarity
check_game_over() is easier to read than 10 lines of if/else code!
Try It Yourself
Challenge 1: Write a function called say_goodbye() that prints “Goodbye!” and “See you later!”.
Solution:
def say_goodbye():
print("Goodbye!")
print("See you later!")
say_goodbye()
Challenge 2: Write a function called square_number(num) that takes a number and returns the number multiplied by itself.
Solution:
def square_number(num):
return num * num
print(square_number(5)) # 25
print(square_number(10)) # 100
Challenge 3: Write a function called make_greeting(name, time) that creates personalized greetings!
Solution:
def make_greeting(name, time):
print("Good", time + ",", name + "!")
print("Hope you're having a great day!")
make_greeting("Elena", "morning")
make_greeting("Sam", "evening")
Extra Challenge: The Calculator
Create a calculator with 4 functions: add, subtract, multiply, divide.
Solution:
def add(x, y):
return x + y
def subtract(x, y):
return x - y
def multiply(x, y):
return x * y
def divide(x, y):
return x / y
print("Calculator Ready!")
num1 = int(input("First number: "))
num2 = int(input("Second number: "))
op = input("Operation (+, -, *, /): ")
if op == "+":
print("Result:", add(num1, num2))
elif op == "-":
print("Result:", subtract(num1, num2))
elif op == "*":
print("Result:", multiply(num1, num2))
elif op == "/":
print("Result:", divide(num1, num2))
Common Mistakes & Fixes
❌ Wrong: def greet()
print("Hi")
Why it fails: Missing the : at the end!
✅ Right: def greet():
print("Hi")
Functions need colons!
❌ Wrong: def greet(name):
print(name)
Why it fails: Not indented!
✅ Right: def greet(name):
print(name)
Indent the function body!
❌ Wrong: def add(x, y)
result = x + y
# Forgot to return!
✅ Right: def add(x, y):
result = x + y
return result
Use return to give back values!
❌ Wrong: greet # Missing ()
Why it fails: Without (), it doesn't run!
✅ Right: greet()
Call with parentheses!
🔍 Bug Hunter Betty Says:
“If you define a function but nothing happens, make sure you CALLED it! Defining is like writing a recipe—you also have to cook it! And if you get ‘NameError’, make sure you defined the function BEFORE you try to use it.”
🎉 Achievement Unlocked: Spell Creator! Wizard Level: MASTER (9/10 complete - one more to go!)
📝 Today’s Spell Book (Quick Reference)
# Define a function (creating the recipe):
def function_name():
# Code goes here
print("This runs when called")
# Call a function (using it):
function_name()
# Function with parameters (ingredients):
def greet(name):
print("Hello", name)
greet("Sam") # Pass the ingredient
# Function with return (giving back a value):
def add(x, y):
return x + y
result = add(5, 3) # result = 8
# Multiple parameters:
def introduce(name, age, hobby):
print(name, "is", age, "and likes", hobby)
introduce("Sam", 10, "gaming")
# Remember:
# - def to define
# - () to call
# - parameters = ingredients
# - return = give back value
# - indent the function body!
💡 Parent/Teacher Tip
Functions are one of the most important concepts in programming! If your learner struggles, use cooking analogies: A recipe (function definition) tells you HOW to make cookies, but you still have to actually MAKE them (function call). Ingredients = parameters, and the finished cookies = return value.
🌟 Fun Fact
Professional programmers spend more time organizing code into functions than writing new code! They follow the “DRY” principle: Don’t Repeat Yourself. If you’re copy-pasting code, you probably need a function!
In the FINAL lesson, we’ll celebrate everything you’ve learned and show you what’s next!
This post is part of the Python Basics for Kids series, based on “Python for Kids from 8 to 88”.
