Python Functions

Defining a function

A function is declared with the def keyword followed by a name and a parameter list.

def greet(name, greeting="Hello"):
    """Return a greeting for the given name."""
    return f"{greeting}, {name}!"

print(greet("Ada"))
print(greet("Ada", greeting="Welcome"))

Argument types

• Positional arguments are matched by order.
• Keyword arguments are matched by name.
• Default arguments supply a fallback value.
• *args collects extra positional arguments into a tuple.
• **kwargs collects extra keyword arguments into a dictionary.

def summarise(*values, **options):
    total = sum(values)
    label = options.get("label", "Total")
    return f"{label}: {total}"

print(summarise(1, 2, 3, label="Score"))

Scope

Names assigned inside a function are local unless declared global or nonlocal. Python resolves names using the LEGB rule: Local, Enclosing, Global, Built-in.

Lambda functions

squares = list(map(lambda x: x * x, range(6)))
print(squares)

Conclusion

Keep functions short, give them one clear responsibility and return values rather than printing them, so they stay testable.