What does return do? Nothing is ever returned

Question

I have been learning Python for some days now. However, I do not understand return. I have read several explanations from my textbooks and online; they don\'t help!

Answer 1

Your textbook expects you to try things in the interactive interpreter, which shows you values as you enter them. Here's an example of how this looks:

$ python
Python 2.7.5+ (default, Sep 17 2013, 17:31:54)
[GCC 4.8.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def sqrt(n):
...     approx = n/2.0
...     better = (approx + n/approx)/2.0
...     while better != approx:
...         approx = better
...         better = (approx + n/approx)/2.0
...     return approx
...
>>> sqrt(25)
5.0
>>>

The key thing here is the difference between expressions and statements. def is a statement, and produces no result. sqrt, which the def block defines, is a function; and functions always produce a return value, such that they can be used in expressions, like sqrt(25). If your function doesn't contain return or yield this value is None, which the interpreter ignores, but in this case sqrt returned a number which is automatically printed (and stored in a variable called _). In a script, you might replace the last line with print sqrt(25) to get the output to a terminal, but the useful thing about return values is that you can do further processing, such as root=sqrt(25) or print sqrt(25)-5.

If we were to run the exact same lines as a script, instead of in interactive mode, there is no implicit printing. The line sqrt(25) is accepted as a statement of an expression, which means it's calculated - but then the value is simply discarded. It doesn't even go into _ (which is the equivalent of the calculator's Ans button). Normally we use this for functions that cause side effects, like quit(), which causes Python to exit.

By the way, print is a statement in Python 2 but a function in Python 3. That's why more and more uses of it have parenthesis.

Here is a script which relies on sqrt (in this case Python's own version) returning a value:

from math import sqrt
area = float(raw_input("Enter a number: "))
shortside = sqrt(area)
print "Two squares with the area", area, "square meters,",
print "placed side to side, form a rectangle", 2*shortside, "meters long",
print "and", shortside, "meters wide"

Answer 2

return returns a value from a function:

def addseven(n):
    return n + 7

a = 9
b = addseven(a)
print(b)        # should be 16

It can also be used to exit a function:

def addseventosix(n):
    if n != 6:
        return
    else:
        return n + 7

However, even if you don't have a return statement in a function (or you use it without specifying a value to return), the function still returns something - None.

def functionthatisuseless(n):
    n + 7

print(functionthatisuseless(8))        # should output None

Sometimes you might want to return multiple values from a function. However, you can't have multiple return statements - control flow leaves the function after the first one, so anything after it won't be executed. In Python, we usually use a tuple, and tuple unpacking:

def addsevenandaddeight(n):
    return (n+7, n+8)        # the parentheses aren't necessary, they are just for clarity

seven, eight = addsevenandaddeight(0)
print(seven)        # should be 7
print(eight)        # should be 8

return statements are what allow you to call functions on results of other functions:

def addseven(n):
    return n+7

def timeseight(n):
    return n*8

print(addseven(timeseight(9))

# what the intepreter is doing (kind of):
# print(addseven(72))    # 72 is what is returned when timeseight is called on 9
# print(79)
# 79

Answer 3

The return keyword is to exit a function and return a value. To let a function return a value, use the return statement.

As Charlie Duffy comments, when you don't use return anywhere in your function, Python has an implicit return value of None. In other words, if you don't tell a function it should return something, then None is what it returns.

So if you change def gimmieFive(): return 5 to just def gimmieFive(): 5, then someone who runs x = gimmieFive() will have x == None, instead of x == 5. The 5 just gets thrown away when gimmieFive exits.

Answer 4

You either need to add a print or write the whole thing into the interactive interpreter to see the return value.

return makes it possible to get some output/result out of a function. To this value later in the code you have to assign it to a variable:

a = sqrt(25)