Functions

A function is a reusable block of code used to perform a specific task.

Defining Python Functions

Use def followed by the function name and parameters:

def print_message(text):
    """Print a message to the console."""
    print(text)

Important rules:

• Function blocks start with def.
• Parameters are written inside parentheses.
• The function body is indented.
• return sends a value back to the caller.
• If no return is given, the function returns None.

Parameters and Arguments

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

print(greet("Bob"))
print(greet("Bob", greeting="Hi"))

Keyword Arguments

def describe_pet(animal, name):
    print(f"I have a {animal} named {name}.")

describe_pet(name="Milo", animal="cat")

Special Parameter Types (Python 3)

Python supports positional-only and keyword-only parameters:

def combine(a, b, /, sep="-", *, upper=False):
    text = f"{a}{sep}{b}"
    return text.upper() if upper else text

print(combine("py", "thon"))
print(combine("py", "thon", sep="_", upper=True))

• Parameters before / are positional-only.
• Parameters after * are keyword-only.

Variable-Length Arguments

def total(*numbers):
    return sum(numbers)


def show_info(**kwargs):
    for key, value in kwargs.items():
        print(key, "=", value)

print(total(1, 2, 3, 4))
show_info(name="Alice", age=22)

lambda (Anonymous Function)

For short one-line functions:

square = lambda x: x * x
print(square(5))

Use def for complex logic to keep code readable.