What Do I Need To Know To Write A Sugar Activity? 

If you are going to write Sugar Activities you should learn something about the topics described in this chapter. There is no need to become an expert in any of them, but you should bookmark their websites and skim through their tutorials. This will help you to understand the code samples we'll be looking at.

Python

Python is the most used language for writing Activities.  While you can use other languages, most Activities have at least some Python in them.  Sugar provides a Python API that simplifies creating Activities.  While it is possible to write Activities using no Python at all (like Etoys), it is unusual. 

Most of the examples in this book are written entirely in Python.  In a later Guest Chapter you'll see how you can mix Python and HTML 5 to make an impressive Activity.

There are compiled languages and interpreted languages. In a compiled language the code you write is translated into the language of the chip it will run on and it is this translation that is actually run by the OS. In an interpreted language there is a program called an interpreter that reads the code you write and does what the code tells it to do. (This is over simplified, but close enough to the truth for this chapter).

Python is an interpreted language. There are advantages to having a language that is compiled and there are advantages to having an interpreted language. The advantages Python has for developing Activities are:

• It is portable. In other words, you can make your program run on any chip and any OS without making a version specific to each one. Compiled programs only run on the OS and chip they are compiled for.
• Since the source code is the thing being run, you can't give someone a Python program without giving them the source code. You can learn a lot about Activity programming by studying other people's code, and there is plenty of it to study.
• It is an easy language for new programmers to learn, but has language features that experienced programmers need.
• It is widely used. One of the best known Python users is Google. They use it enough that they have a project named “Unladen Swallow” to make Python programs run faster.

The big advantage of a compiled language is that it can run much faster than an interpreted language. However, in actual practice a Python program can perform as well as a compiled program. To understand why this is you need to understand how a Python program is made.

Python is known as a “glue” language. The idea is that you have components written in various languages (usually C and C++) and they have Python bindings. Python is used to “glue” these components together to create applications. In most applications the bulk of the application's function is done by these compiled components, and the application spends relatively little time running the Python code that glues the components together.

In addition to Activities using Python most of the Sugar environment itself is written in Python.

If you have programmed in other languages before there is a good tutorial for learning Python at the Python website: http://docs.python.org/tutorial/.  If you're just starting out in programming you might check out Invent Your Own Computer Games With Python, which you can read for free at http://inventwithpython.com/.

PyGTK

GTK+ is a set of components for creating user interfaces. These components include things like buttons, scroll bars, list boxes, and so on. It is used by GNOME desktop environment and the applications that run under it. Sugar Activities use a special GNOME theme that give GTK+ controls a unique look.

PyGTK is a set of Python bindings that let you use GTK+ components in Python programs. Sugar 3 uses GTK+ 3 and Activities written for it should also, although the previous version of PyGTK still works. GTK+3 is very different from what went before. There is a tutorial showing how to use it at the PyGTK website: http://python-gtk-3-tutorial.readthedocs.org/en/latest/. You'll also find an article on converting Activities using GTK 2 to use GTK 3 in the guest chapters section of this book.

PyGame

The alternative to using PyGTK for your Activity is PyGame. PyGame can create images called sprites and move them around on the screen. As you might expect, PyGame is mostly used for writing games. It is less commonly used in Activities than PyGTK.

The tutorial to learn about PyGame is at the PyGame website: http://www.pygame.org/wiki/tutorials. The website also has a bunch of pygame projects you can download and try out.