Python Lesson 10: Functions

A function is a reusable block of code that does one specific job. Define it once, call it anywhere. Functions are the building blocks of every program.

Defining and Calling a Function

# Define the function with def
def greet():
    print("Hello! Welcome to Python!")
    print("Learning to code is awesome!")

# Call it by name
greet()    # runs the code inside
greet()    # can call it many times!

Functions with Parameters

def greet_person(name):
    print(f"Hello, {name}! Nice to meet you!")

greet_person("Alice")   # Hello, Alice!
greet_person("Bob")     # Hello, Bob!

def add(a, b):
    print(f"{a} + {b} = {a + b}")

add(5, 3)    # 5 + 3 = 8
add(10, 20)  # 10 + 20 = 30

return — Get Values Back

def add(a, b):
    return a + b   # sends the result back to the caller

result = add(5, 3)
print(result)            # 8
print(add(10, 20) * 2)  # 60

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

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

Default Parameters

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

greet("Alice")               # Hello, Alice!
greet("Bob", "Good morning") # Good morning, Bob!

Variable Scope

x = 10   # global variable

def my_function():
    y = 20   # local variable (only inside function)
    print(x)  # can read global
    print(y)  # can read local

my_function()
# print(y)  ERROR! y does not exist outside function