Page 44 - Programming the Raspberry Pi Getting Started with Python

Figure 4-2 A complex list

You can combine what you know about lists with for loops and write a short program that creates a
list and then prints out each element of the list on a separate line:

Here’s the output of this program:

Functions

When you are writing small programs like the ones we have been writing so far, they only really
perform one function, so there is little need to break them up. It is fairly easy to see what they are
trying to achieve. As programs get larger, however, things get more complicated and it becomes
necessary to break up your programs into units called functions. When we get even further into
programming, we will look at better ways still of structuring our programs using classes and modules.
Many of the things I have been referring to as commands are actually functions that are built into
Python. Examples of this are range and print.
The biggest problem in software development of any sort is managing complexity. The best
programmers write software that is easy to look at and understand and requires very little in the way
of extra explanation. Functions are a key tool in creating easy-to-understand programs that can be
changed without difficulty or risk of the whole thing falling into a crumpled mess.
A function is a little like a program within a program. We can use it to wrap up a sequence of
commands we want to do. A function that we define can be called from anywhere in our program and
will contain its own variables and its own list of commands. When the commands have been run, we
are returned to just after wherever it was in the code we called the function in the first place.
As an example, let’s create a function that simply takes a string as an argument and adds the word
please to the end of it. Load the following file—or even better, type it in to a new Editor window—and

then run it to see what happens: