Functions

A function is a reusable block of code that performs a specific task.

Defining a Function

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

greet()

Output:

Hello!

Parameters

Parameters let you pass data into a function:

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

greet("Alice")
greet("Bob")

Output:

Hello, Alice!
Hello, Bob!

Multiple Parameters

def greet(name, time_of_day):
    print("Good " + time_of_day + ", " + name + "!")

greet("Alice", "morning")

Output:

Good morning, Alice!

Return Values

Use return to send a value back from a function:

def add(a, b):
    return a + b

result = add(3, 5)
print(result)

Output:

Note: If a function has no return, it returns None.

Default Parameters

Give parameters default values:

def greet(name, greeting="Hello"):
    print(greeting + ", " + name + "!")

greet("Alice")
greet("Bob", "Hi")

Output:

Hello, Alice!
Hi, Bob!

Scope

Variables inside a function are local—they only exist inside that function:

def calculate():
    x = 10
    return x * 2

result = calculate()
print(result)  # 20
print(x)       # Error! x doesn't exist here

Functions Calling Functions

Functions can call other functions:

def square(n):
    return n * n

def sum_of_squares(a, b):
    return square(a) + square(b)

print(sum_of_squares(3, 4))

Output:

Documenting Functions

Use a docstring to describe what a function does:

def calculate_area(width, height):
    """Calculate the area of a rectangle."""
    return width * height

Practical Examples

Processing a list

def get_average(numbers):
    total = 0
    for num in numbers:
        total = total + num
    return total / len(numbers)

scores = [85, 90, 78, 92]
print(get_average(scores))

Output:

86.25

Validating input

def is_valid_age(age):
    if age < 0:
        return False
    if age > 120:
        return False
    return True

print(is_valid_age(25))   # True
print(is_valid_age(-5))   # False

Formatting output

def format_name(first, last):
    return last + ", " + first

print(format_name("Alice", "Smith"))

Output:

Smith, Alice

Common Mistakes

Forgetting to call the function

# Wrong - this just references the function
def greet():
    print("Hello!")

greet  # Nothing happens

# Right - use parentheses to call it
greet()

Forgetting to return a value

# Wrong - returns None
def add(a, b):
    result = a + b

print(add(3, 5))  # None

# Right - return the result
def add(a, b):
    return a + b

print(add(3, 5))  # 8

Printing instead of returning

# Wrong - can't use the result
def add(a, b):
    print(a + b)

result = add(3, 5)  # result is None

# Right - return so you can use it
def add(a, b):
    return a + b

result = add(3, 5)  # result is 8

Wrong number of arguments

def greet(name, age):
    print(name + " is " + str(age))

# Wrong - missing argument
greet("Alice")  # Error!

# Right - provide both arguments
greet("Alice", 20)

Indentation errors

# Wrong - code outside the function
def greet():
print("Hello!")

# Right - indent code inside the function
def greet():
    print("Hello!")