Think Python, 3rd Edition

Chapter 3. Functions

In the previous chapter we used several functions provided by Python, like int and float, and a few provided by the math module, like sqrt and pow. In this chapter, you will learn how to create your own functions and run them. And we’ll see how one function can call another. As examples, we’ll display lyrics from Monty Python songs. These silly examples demonstrate an important feature—the ability to write your own functions is the foundation of programming.

This chapter also introduces a new statement, the for loop, which is used to repeat a computation.

Defining New Functions

A function definition specifies the name of a new function and the sequence of statements that run when the function is called. Here’s an example:

def print_lyrics():
    print("I'm a lumberjack, and I'm okay.")
    print("I sleep all night and I work all day.")
       

def is a keyword that indicates that this is a function definition. The name of the function is print_lyrics. Anything that’s a legal variable name is also a legal function name.

The empty parentheses after the name indicate that this function doesn’t take any arguments.

The first line of the function definition is called the header—the rest is called the body. The header has to end with a colon and the body has to be indented. By convention, indentation is always four spaces. The body of this function is two print statements; in general, the body of a function can contain any number of statements of any kind.

Defining a function ...