Python: Functions

What you will learn

How to define a reusable block of code, pass arguments, return values, and use default parameters.

Defining and calling a function

def greet(name):
    """Return a greeting string."""
    return f"Hello, {name}!"

print(greet("Alice"))  # Hello, Alice!

Functions are defined with def, followed by the name, parentheses for parameters, and a colon. The body is indented. The optional return sends a value back where the function was called.

Default arguments

def power(base, exponent=2):
    return base ** exponent

print(power(3))     # 9  (exponent defaults to 2)
print(power(3, 3))  # 27

Parameters with defaults must come after parameters without defaults.

Multiple return values

def min_max(nums):
    return min(nums), max(nums)

lo, hi = min_max([1, 5, 2, 8])
print(lo, hi)  # 1 8

Python packs multiple values into a tuple automatically.

Common mistakes

• Mutable default arguments (def f(x=[])) are shared across calls. Use None and create a new list inside instead.
• Forgetting the colon or mis-indenting the body.

Quick check below!