Return a Value

Some functions perform an action, such as displaying a message. Other functions calculate or build a value for the rest of the program to use.

You have already used that second kind:

letter_count = len("Python")

len("Python") produces the integer 6, and the assignment stores it in letter_count.

Your own functions can produce values with return.

def double(number):
    return number * 2

The function call now has a value:

result = double(7)
print(result)

The argument 7 becomes the parameter number. The expression number * 2 produces 14. Then return sends 14 back to the caller, where it is assigned to result.

A call can be used as an expression

Because double(7) returns a value, you can use the call anywhere that value would make sense:

Assigning the result to a variable is often easiest to read, especially while you are learning. The compact forms are useful when they remain clear.

Return ends the call

When Python reaches return, it immediately sends the value back to the caller. Lines later in that function body do not run:

def double(number):
    return number * 2
    print("This line is unreachable.")

For now, put the main calculation in one return near the end of a small function. Later chapters will show situations where an earlier return makes sense.

Return information, then decide how to use it

Here is a function that builds text without deciding where that text should go:

def make_label(name, place):
    return f"{name} -> {place}"

One caller can print the label:

print(make_label("Iris", "Oslo"))

Another can store it:

first_trip = make_label("Iris", "Oslo")

Another can put it in a list:

trips = [make_label("Iris", "Oslo"), make_label("Dev", "Accra")]

The function stays useful because it returns the result instead of making every later decision itself.

def add_fee(price):
    return price + 3

total = add_fee(7)
print(total * 2)