Print or Return?

Beginners often mix up return and print(), because both seem to give a value out. But they do very different things:

• print() only shows a value to you, in the output. The program can't work with what was printed.
• return gives the value to the program, which can store it, calculate with it or print it.

Look at what happens when a function prints its result instead of returning it:

def print_sum(a, b):
    print(a + b)

result = print_sum(2, 3)
print(result)

This prints:

5
None

The 5 was printed by the print() inside the function. But the function didn't return anything, and a function without return returns None: a special value in Python that means “nothing”. So result is None, and the second print() prints None.

When you try to calculate with None, the program ends with an error:

def print_sum(a, b):
    print(a + b)

total = print_sum(2, 3) * 2

This prints:

5
Traceback (most recent call last):
  File "<exec>", line 4, in <module>
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

The error message says that * can't work with None (whose type is NoneType) and a whole number (int). Whenever you see None or NoneType where you expected a value, check whether your function has a return.

It can also happen the other way round: a function returns a value, but nobody uses it:

def add(a, b):
    return a + b

add(2, 3)

This program prints nothing. The function returned 5, but the program didn't store it or print it, so the value was lost. If you want to see it, print it: print(add(2, 3)).

So when should a function print, and when should it return? A function that calculates something should return the result. The code that calls it then decides what to do with it: print it, store it or keep calculating with it. Print inside a function only when showing something is its whole job, like in say_hello() or print_area().