Python Functions

What Are Functions?

A function is a reusable block of code that performs a specific task. Instead of writing the same code over and over, you define a function once and call it whenever you need it. You have already used built-in functions like print() and len() -- now you will learn to create your own.

Functions help keep your code organised, readable, and DRY (Don't Repeat Yourself). In Python, you define a function with the def keyword:

# Defining a simple function
def greet():
    print("Hello! Welcome to BrightPath.")

# Calling the function
greet()          # Output: Hello! Welcome to BrightPath.
greet()          # You can call it as many times as you like

The indented code under def is the function body. It only runs when you call the function by writing its name followed by parentheses.

Parameters and Arguments

Functions become much more powerful when you give them parameters -- placeholders for data that the function will use. When you call the function, you pass arguments (actual values) into those parameters.

# Function with parameters
def greet_student(name, year):
    print(f"Hello {name}! Welcome to Year {year}.")

greet_student("Mia", 10)    # Output: Hello Mia! Welcome to Year 10.
greet_student("Liam", 9)    # Output: Hello Liam! Welcome to Year 9.

# Default parameter values
def power(base, exponent=2):
    return base ** exponent

print(power(5))       # Output: 25   (uses default exponent=2)
print(power(5, 3))    # Output: 125  (exponent overridden to 3)

You can set default values for parameters. If the caller does not provide a value, the default is used. Parameters with defaults must come after parameters without defaults.

Return Values and Variable Scope

The return statement sends a value back to the caller. Without a return statement, a function returns None by default.

# Function that returns a value
def calculate_area(length, width):
    area = length * width
    return area

result = calculate_area(8, 5)
print(f"The area is {result} square units.")  # Output: The area is 40 square units.

# Variable scope: local vs global
total = 0               # Global variable

def add_score(points):
    bonus = 10           # Local variable -- only exists inside this function
    return points + bonus

print(add_score(50))     # Output: 60
# print(bonus)           # ERROR! 'bonus' is not defined outside the function

Scope determines where a variable can be accessed. Variables created inside a function are local -- they exist only while the function runs. Variables created outside all functions are global and can be read anywhere, but modifying them inside a function requires the global keyword.

Key Vocabulary

Worked Examples

def show_result(name, score):
    if score >= 50:
        print(f"Well done {name}! You passed with {score}%.")
    else:
        print(f"Keep trying {name}. You scored {score}%.")

show_result("Ava", 85)   # Output: Well done Ava! You passed with 85%.
show_result("Tom", 42)   # Output: Keep trying Tom. You scored 42%.

def average(a, b, c):
    total = a + b + c
    return total / 3

result = average(80, 90, 70)
print(f"Average: {result}")    # Output: Average: 80.0

school = "BrightPath"      # Global variable

def welcome():
    greeting = "Welcome to"  # Local variable
    return f"{greeting} {school}"

print(welcome())           # Output: Welcome to BrightPath
print(school)              # Output: BrightPath
# print(greeting)          # ERROR -- greeting is local to welcome()

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

def double(n):
    return n * 2

result = double(7)