lesson 43 Functions · beginner

Return a result

return hands a value back to the caller and ends the function. A function without return gives back None, which is why printing is not the same as returning.

You'll be able to

Return a computed value and use the call inside an expression
Return early from inside an if or a loop, and return None when nothing is found
Explain the difference between printing a value and returning it

The idea

A function that prints shows a value to a person, but the program can't use it: it can't add it to a total or compare it. return value hands the value back to the code that called the function. The call itself then is that value, so line_total(2, 6.49) + 5 works like any other sum.

return also ends the function at once. A function that reaches its end without a return gives back None.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Line totals the program can add up

def line_total(quantity, unit_price):
    return quantity * unit_price

first = line_total(2, 6.49)
second = line_total(1, 51.49)
subtotal = first + second
print(f"{first:.2f} + {second:.2f} = {subtotal:.2f}")
print(line_total(3, 10) > 25)

example-2.py · Printing instead of returning

def shipping_fee(subtotal):
    if subtotal >= 50:
        print(0)
    else:
        print(5.99)

total = 12.48 + shipping_fee(12.48)

A function can have several return lines; the first one reached wins. That makes decisions read clearly: test a case, return its answer, and the code below only runs for the other cases. Inside a loop, return stops the search as soon as the answer is known, and a return None after the loop says *not found*.

example-3.py · Shipping rules and the first late order

def shipping_fee(subtotal, country):
    if country != "US":
        return 12.0
    if subtotal >= 50:
        return 0
    return 5.99

def first_late(deliveries, limit):
    for order_id, days in deliveries:
        if days > limit:
            return order_id
    return None

print(shipping_fee(64.47, "US"), shipping_fee(12.48, "US"), shipping_fee(80, "CA"))
deliveries = [("O100043", 3), ("O101217", 9), ("O108752", 12)]
print(first_late(deliveries, 7), first_late(deliveries, 20))

When to use it: almost always. Return the result and let the caller print it; a function that returns can be printed, stored, tested and reused, while a function that prints can only be read. Mistakes to avoid:

Printing inside the function when the caller needs the value.
Putting return inside a loop when every item must be checked first: it stops on the first pass.
Forgetting the return None (or a final return) for the case where nothing matches.

Review

return value hands the value to the caller and ends the function; no return means None.
Several returns make decisions clear; a return in a loop stops the search at the first match.
Return results and let the caller print them.
Next: default values for parameters, and the names that live only inside a function.

Practise this