Functions

So far my description of modularization may seem hopelessly abstract. What do these modules look like? In Python the simplest form of modularization is to create functions. We have already used many of Python's built-in functions, e.g. range(), len(), math.sqrt(), and random.randint(), and now it is time to learn to write our own functions.

There is a bit of terminology that will help us talk about functions. We say that “the function len() when called, returns an integer specified by its argument”. What do the terms called, return, and argument refer to? Consider the specific statement,

mismash = [0, True, 3.8, "Don't Panic"]

print(len(mismash))

To call a function is to invoke it and we do that by entering its name followed by parentheses. The name of the function being used here is len. To do its job it needs to find the length of a list. We tell it which one by providing the list we need to know the length of. This value is the argument to the function. It is sometimes also called the function parameter. The function len will calculate the length of the list 4. This is the value it returns or its return value. You can imagine the return value replacing the function call so the code above is equivalent to

print(4)

So every function has,

• a name
• a list of parameters enclosed in parentheses after the name of the function (this list may be "empty", i.e. there may be no values specified, but the parentheses must still be present to mark the name as specifying a function rather than a variable), and
• a return value, the value that will replace the expression calling the function (this value may be None if the function does something rather than calculating something).