Python Functions — Writing Your Own Tools

A function packages logic so you can call it by name anywhere. Name it clearly and your code reads like a story. Name it 'do_stuff' and your future self will curse your past self.

def greet(name: str, enthusiasm: int = 1) -> str:
    """Return a friendly greeting.

    Args:
        name: the person to greet
        enthusiasm: number of exclamation marks (default 1)
    """
    bangs = "!" * enthusiasm
    return f"Hello, {name}{bangs}"

print(greet("Maya"))
print(greet("Maya", enthusiasm=5))

Parameters, arguments, defaults

• A parameter is the name in the definition (name, enthusiasm)
• An argument is the value you pass when calling ("Maya", 5)
• Defaults let callers skip arguments — use them to keep simple calls simple
• Keyword arguments (enthusiasm=5) make calls self-documenting

Return vs. print — the classic beginner mix-up

def add_broken(a, b):
    print(a + b)           # shows the answer — but returns None

def add_good(a, b):
    return a + b           # gives the answer back to the caller

result = add_broken(2, 3)  # prints 5, but result is None
total = add_good(2, 3)     # total is 5 — usable

print(result * 2)          # TypeError: NoneType
print(total * 2)           # 10

Mini-exercise: price calculator

1. 1Write calc_total(items: list[float], tax_rate: float = 0.08) -> float
2. 2Return the sum of items times (1 + tax_rate), rounded to 2 decimals
3. 3Add a docstring
4. 4Call it with [3.50, 4.25, 1.99] and print the result
5. 5Ask your AI assistant to review your code and suggest improvements

Big idea: a function is a small contract. Its name says what it does, its signature says how to use it, its docstring says why. AI pair-programming gets dramatically better when you tighten those three things up first.