Print vs. return

Section 4.12 Print vs. return

Many beginning programmers find the distinction between a non-fruitful-, void-fuction that prints the results, and a fruitful function that returns a value, very confusing. This is especially difficult since most of the illustrations of return values in intro texts like this one show the returned value from a function call by printing it, as in print(square(g(2))).

The print statement is fairly easy to understand. It takes a Python object and outputs a printed representation of it in the output window. You can think of the print statement as something that takes an object from the land of the program and makes it visible to the land of the human observer/program user.

Note 4.12.1.

Print is for people. Remember that slogan. Printing has no effect on the ongoing execution of a program. It doesn’t assign a value to a variable. It doesn’t return a value from a function call.

Return is for reuse. When you, the programmer, need to use or reuse a value determined by a function in a later step of your program, you should assign it to a variable. Ensure the function’s return value goes to a variable.

If you’re confused, chances are the source of your confusion is really about returned values and the evaluation of complex expressions. A function that returns a value is producing a value for use by the program, in particular for use in the part of the code where the function was invoked. Remember that when a function is invoked, the function’s code block is executed – all that code indented under the def statement gets executed, following the rules of the Python language for what should and should not execute as it flows through the program. But when the function returns, control goes back to the calling location, and a return value can come back with it.

You’ve seen some function calls in earlier chapters. When we told you about the function square that we defined, you saw that the expression square(2) evaluated to the integer value 4.

That’s because the square function returns a value: the square of whatever input is passed into it.

What do you use the result of a function call for? There are many possibilities.

Save it for later.
The returned value may be:
¹
The last two possibilities, data structures and files, will be discussed in later chapters
Assigned to a variable. For example, w = square(3)

Put into a data structure like a list. For example, L.append(square(3))

Sent to a file. For example, someFile.write(square(3))
Use it in a more complex expression.

In that case, think of the return value as replacing the entire text of the function invocation. For example, if there is a line of code w = square(square(3) + 7) - 5, think of the return value 9 replacing the text square(3) in that invocation, so it becomes square(9 + 7) -5.
Print it for human consumption.

For example, print(square(3)) outputs 9 to the output area. Note that, unless the return value is first saved as in possibility 1, it will be available only to the humans watching the output area, not to the program as it continues executing.

If your only purpose in running a function is to make an output visible for human consumption, there are two ways to do it. You can put one or more print statements inside the function definition and not bother to return anything from the function (the value None will be returned). In that case, invoke the function without a print statement. For example, you can have an entire line of code that reads f(3). That will run the function f and throw away the return value. Of course, if square doesn’t print anything out, it’s useless to call it and do nothing with the return value. But with a function that has print statements inside it, it can be quite useful.

The other possibility is to return a value from the function and print it, as in print(f(3)). As you start to write larger, more complex programs, this will be more typical. Indeed the print statement will usually only be a temporary measure while you’re developing the program. Eventually, you’ll end up calling f and saving the return value or using it as part of a more complex expression.

You will know you’ve really internalized the idea of functions when you are no longer confused about the difference between print and return. Keep working at it until it makes sense to you!

Check your understanding

Checkpoint 4.12.2.

What will the following code output?

def square(x):
    return x*x

def g(y):
    return y + 3

def h(y):
    return square(y) + 3

print(h(2))

2
2 is the input; the value returned from h is what will be printed.
5
Don’t forget that 2 gets squared.
7
First square 2, then add 3.
25
3 is added to the result of squaring 2
Error: y has a value but x is an unbound variable inside the square function
When square is called, x is bound to the parameter value that is passed in, 2.

Checkpoint 4.12.3.

What will the following code output?

def square(x):
    return x*x

def g(y):
    return y + 3

def h(y):
    return square(y) + 3

print(g(h(2)))

2
Better read the section above one more time.
5
Better read the section above one more time.
7
That’s h(2), but it is passed to g.
10
h(2) returns 7, so y is bound to 7 when g is invoked.
Error: you can’t nest function calls
Ah, but you can nest function calls.

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