Functions

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


def stats(numbers):
    return min(numbers), max(numbers), sum(numbers) / len(numbers)


def main():
    print(greet("Ada"))
    print(greet("Linus", greeting="Hi"))
    low, high, mean = stats([4, 8, 6])
    print(low, high, mean)
    square = lambda x: x * x
    print(square(9))


if __name__ == "__main__":
    main()

Hello, Ada!
Hi, Linus!
4 8 6.0
81

• A function without return returns None.
• Returning several values returns a tuple, which you can unpack: low, high, mean = ....
• if __name__ == "__main__": runs main() only when this file is the one being run, not when another file imports it.

Watch out: Never use a mutable default such as def add(item, items=[]): the same list is shared by every call. Use items=None and create the list inside.