def, Parameters, and return

Scenario: The same health calculation appears in five scripts.

New words, in plain English

• Function - a named reusable block of code
• Parameter - an input name in a function definition
• Argument - the actual value supplied in a call
• return - send a result back to the caller

Define with def. A function call creates local parameter names. return ends the call; without an explicit return, Python returns None.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

def healthy(status_code):
    return 200 <= status_code < 400

print(healthy(204))

Tip: Prefer functions that return useful values over functions that only print.

Goal: Practice this idea in the py-first-function challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.