Python Functions

Defining a function

A function is a reusable block of code. Define it with def, give it parameters, and return a result.

def greet(name):
    """Return a friendly greeting."""
    return f"Hello, {name}!"

message = greet("Ada")
print(message)

def log(text):
    print("LOG:", text)

result = log("started")
print(result)

Hello, Ada!
LOG: started
None

A function without return returns None.

Why functions?

• Reuse: write logic once, call it many times.
• Naming: a good function name explains what the code does.
• Testing: small functions are easy to test.

Functions are objects

You can store functions in variables, pass them to other functions and return them — this is what makes decorators and callbacks possible.

def shout(text):
    return text.upper() + "!"

def apply(func, value):
    return func(value)

print(apply(shout, "hi"))
print(apply(len, "hello"))

HI!
5