o_18ufhmfmq19t513t3lgmn5l1qa8a.pdf

112 CHAPTER 6 ■ ABSTRACTION
■Tip Your functions can return more than one value—simply collect them in a tuple and return that.
Documenting Functions
If you want to document your functions so that you’re certain that others will understand them
later on, you can add comments (beginning with the hash sign, #). Another way of writing
comments is simply to write strings by themselves. Such strings can be particularly useful in
some places, such as right after a def statement (and at the beginning of a module or a class—
you learn more about those later in the book). If you put a string at the beginning of a function,
it is stored as part of the function and is called a docstring. The following code demonstrates
how to add a docstring to a function:
def square(x):
'Calculates the square of the number x.'
return x*x
The docstring may be accessed like this:
>>> square.__doc__
'Calculates the square of the number x.'
■Note __doc__ is a function attribute. You’ll learn a lot more about attributes in Chapter 7. The double
underscores in the attribute name mean that this is a special attribute. Special or “magic” attributes like this
are discussed in Chapter 9.
There is a built-in function called help, which can be quite useful. If you use it in the interactive
interpreter, you can get information about a function, including its docstring:
>>> help(square)
Help on function square in module __main__:
square(x)
Calculates the square of the number x.
You meet the help function again in Chapter 10.
Functions That Aren’t Really Functions
Functions, in the mathematical sense, always return something that is calculated from their
parameters. In Python, some functions don’t return anything. In other languages (such as Pascal),
such functions may be called other things (such as procedures), but in Python a function is a
function, even if it technically isn’t. Functions that don’t return anything simply don’t have a
return statement. Or, if they do have return statements, there is no value after the word return: