Functions Explained¶

Try This First: Before reading, try writing a function that adds two numbers: def add(a, b): return a + b. Then call it: print(add(3, 4)). What does it print?

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A function is a reusable block of code with a name. You define it once, then call it whenever you need it.

Visualize It¶

See how Python enters a function, uses parameters, and returns a value: Open in Python Tutor

Defining a function¶

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

def means "I am defining a function"
greet is the name you chose
name is a parameter — a value the function expects to receive
return sends a value back to whoever called the function

Calling a function¶

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

# Or use it directly
print(greet("Alice"))  # Hello, Alice!

Functions without return¶

Some functions do something (like printing) but do not return a value:

def say_hello():
    print("Hello!")

say_hello()  # Prints: Hello!

If there is no return, the function returns None automatically.

Parameters and arguments¶

Parameters are the names in the function definition. Arguments are the values you pass when calling.

def add(a, b):     # a and b are parameters
    return a + b

add(3, 5)          # 3 and 5 are arguments

Default values¶

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

greet("Alice")           # Hello, Alice!
greet("Alice", "Hey")    # Hey, Alice!

Why functions matter¶

Reuse — write code once, use it everywhere
Organize — break big problems into named pieces
Test — test each piece independently
Read — calculate_tax(price) is clearer than 5 lines of math

Common mistakes¶

Forgetting parentheses when calling:

greet          # This is the function OBJECT, not a call
greet("Alice") # This CALLS the function

Forgetting to return:

def add(a, b):
    result = a + b
    # Forgot to return result!

total = add(3, 5)  # total is None, not 8

Defining after calling:

greet("Alice")    # Error! greet is not defined yet

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

Practice¶

Quick check: Take the quiz

Review: Flashcard decks Practice reps: Coding challenges

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