Python in a Nutshell, 2nd Edition

Functions

Most statements in a typical Python program are grouped and organized into functions (code in a function body may be faster than at a module’s top level, as covered in Avoiding exec and from...import *, so there are excellent practical reasons to put most of your code into functions). A function is a group of statements that execute upon request. Python provides many built-in functions and allows programmers to define their own functions. A request to execute a function is known as a function call. When you call a function, you can pass arguments that specify data upon which the function performs its computation. In Python, a function always returns a result value, either None or a value that represents the results of the computation. Functions defined within class statements are also known as methods. Issues specific to methods are covered in Bound and Unbound Methods; the general coverage of functions in this section, however, also applies to methods.

In Python, functions are objects (values) that are handled like other objects. Thus, you can pass a function as an argument in a call to another function. Similarly, a function can return another function as the result of a call. A function, just like any other object, can be bound to a variable, an item in a container, or an attribute of an object. Functions can also be keys into a dictionary. For example, if you need to quickly find a function’s inverse given the function, you could define a dictionary whose keys and values ...