Python Functions

A function is a named block of code that can receive inputs and return a result. Functions make programs easier to test, reuse, and read. Define one with def, then call it by name. You will also separate calculation from display and validate at the function boundary.

Define and call

The definition creates the function but does not run it. Calling the function with parentheses runs the body. Parameters are the names in the definition; arguments are the values you pass in.

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

print(greet("Maya"))

A reusable message

Return a value

return sends a value back to the caller and ends the function. print only displays text; it does not give the caller a usable result. A function with no return statement returns None.

def total(price, quantity):
    return price * quantity

result = total(4.5, 2)
print(result)

Returning a calculation

Defaults and keyword arguments

A default parameter is used when the caller omits that argument. Parameters with defaults must come after parameters without them. Keyword arguments make calls readable when a function has several parameters.

def total(price, quantity=1):
    return price * quantity

print(total(4.5))
print(total(price=4.5, quantity=3))

Defaults and named arguments

Return, don't print

A function that prints is hard to reuse; a function that returns can be combined, tested, and formatted by the caller. Keep calculation and presentation separate. Print at the edge of the program, not in the middle of the calculation.

def line_total(qty, price):
    return round(qty * price, 2)

subtotal = line_total(2, 4.5) + line_total(1, 3.0)
print(f"subtotal: {subtotal:.2f}")

A return value can be composed

Validate at the boundary

Validate inputs where they enter the function, and raise a specific exception with a useful message. The caller can then decide whether to retry, skip the row, or stop. A function that silently returns None hides the failure.

def line_total(qty, price):
    if qty <= 0:
        raise ValueError("qty must be positive")
    if price < 0:
        raise ValueError("price must be non-negative")
    return round(qty * price, 2)

print(line_total(2, 4.5))
try:
    line_total(0, 4.5)
except ValueError as error:
    print(type(error).__name__, error)

Validate once, fail clearly

Never use a mutable default

Default arguments are evaluated once, when the function is defined. A list default is therefore shared by every call that does not pass its own list. Use None as the default and create a new list inside the function. This is not a style preference; it prevents data from leaking between calls.

def add_item(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

print(add_item("latte"))
print(add_item("tea"))

A fresh list for each call

Try it yourself

Change the values, predict the output, then run the code. The Python runtime is downloaded the first time you press Run, so the first run can take a few seconds.