Functions, *args, **kwargs

Reusable blocks. *args collects positional args into a tuple; **kwargs collects keyword args into a dict.

Code example

def greet(name, greeting='Hello', **extras):
    msg = f"{greeting}, {name}!"
    if extras.get('shout'):
        msg = msg.upper()
    return msg

greet('Ada')                       # 'Hello, Ada!'
greet('Ada', greeting='Hi')        # 'Hi, Ada!'
greet('Ada', greeting='Hi', shout=True)  # 'HI, ADA!'

How it works

Functions are defined with `def` and return with `return` (None if no return statement). Default arguments are evaluated once at function-definition time — mutable defaults like `def f(x=[])` are a famous footgun. `*args` collects extra positional args into a tuple; `**kwargs` collects extra keyword args into a dict. FastAPI uses `**kwargs` heavily in route signatures — every query param, path param, and body becomes a kwarg.