def names a reusable block. Arguments go in parentheses. return sends a value back. Without return, the function returns None.

Goal

Define functions with arguments, defaults, and a return value.

A first function

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

print(greet("Ada"))
print(greet("Alan"))

Defaults

def band(score, pass_mark=90):
    if score >= pass_mark:
        return "pass"
    return "retry"

print(band(91))
print(band(88))
print(band(88, pass_mark=80))

Default values are bound once. Do not use a mutable default like [] — pass None and create a list inside.

Several results

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

lo, hi, avg = stats([98, 91, 95, 88])
print(lo, hi, round(avg, 1))

`*args`

def total(*nums):
    running = 0
    for n in nums:
        running += n
    return running

print(total(1, 2, 3, 4))
print(total(*[10, 20, 30]))

* in a call unpacks a list into arguments.

Docstring

def celsius_to_f(c):
    """Convert Celsius to Fahrenheit."""
    return c * 9 / 5 + 32

print(celsius_to_f(0))
print(celsius_to_f.__doc__)

Tip

Name functions with verbs: greet, band, total. Names that are only nouns often belong to data (lists, dicts, classes).