Introducing Python, 2nd Edition

Chapter 4. Choose with if

If you can keep your head when all about you Are losing theirs and blaming it on you, …

Rudyard Kipling, If—

In the previous chapters, you’ve seen many examples of data but haven’t done much with them. Most of the code examples used the interactive interpreter and were short. In this chapter, you learn how to structure Python code, not just data.

Many computer languages use characters such as curly braces ({ and }) or keywords such as begin and end to mark off sections of code. In those languages, it’s good practice to use consistent indentation to make your program more readable for yourself and others. There are even tools to make your code line up nicely.

When he was designing the language that became Python, Guido van Rossum decided that the indentation itself was enough to define a program’s structure, and avoided typing all those parentheses and curly braces. Python is unusual in this use of white space to define program structure. It’s one of the first aspects that newcomers notice, and it can seem odd to those who have experience with other languages. It turns out that after writing Python for a little while, it feels natural, and you stop noticing it. You even get used to doing more while typing less.

Our initial code examples have been one-liners. Let’s first see how to make comments and multiple-line commands.

Comment with #

A comment is a piece of text in your program that is ignored by the Python interpreter. You might use comments to ...