Functions

Everything you have written so far runs straight down the page. That works, but the moment the same piece of work appears in two places, a fix means fixing both. Three places, three fixes. Writing the same thing over and over is the thickest source of bugs there is.

A function turns that source off. You give a chunk of work a name, and call it by name when you want it. There is one place to change, and the name tells the reader what it does, so the code gets easier to follow.

In Python you make one with def. Write def greet(): and put the body indented underneath. This is where the indenting from Chapter 1 really starts to earn its keep — how deep the whitespace goes is the only thing deciding where the function ends.

You can hand values in (parameters) and get a value back (a return value). def add(a, b): takes two values in, and return a + b sends the answer back to whoever called it. Once "hand in, hand back" clicks, you can start writing programs as parts that fit together.

The most important thing in this chapter is probably the difference between print and return. Both look like "something comes out of the function", and they are nothing alike. print is for a human to look at; return is for the program to use. A printed value cannot go into another calculation, and a returned value never appears on screen unless you ask for it. Carry the two muddled together and you will hit a wall later. We spend a whole lesson on it.

Scope comes up too. A variable made inside a function cannot be seen from outside. That feels like an obstacle at first, but it is looking after you: an i inside a function can never quietly wreck an i outside it.

Later in the chapter come keyword arguments and defaults. You can name a value as you pass it, greet(name="Yui"), and you can decide what to use when nothing is passed, def greet(name="guest"):. Python's libraries are written assuming you know this, so learning it suddenly makes other people's code far easier to read.