Before moving on, we should point out that the internal execution flow described in the prior section reflects the standard implementation of Python today, and is not really a requirement of the Python language itself. Because of that, the execution model is prone to change with time. In fact, there are already a few systems that modify the picture in Figure 2-2 somewhat. Let’s take a few moments to explore the most prominent of these variations.

Really, as this book is being written, there are two primary implementations of the Python language— CPython and Jython—along with a handful of secondary implementations such as Python.net. CPython is the standard implementation; all the others have very specific purposes and roles. All implement the same Python language, but execute programs in different ways.

The original, and standard, implementation of Python is usually called CPython, when you want to contrast it with the other two. Its name comes from the fact that it is coded in portable ANSI C language code. This is the Python that you fetch from www.python.org, get with the ActivePython distribution, and have automatically in most Linux machines. If you’ve found a preinstalled version of Python on your machine, it’s probably CPython as well, unless your company is using Python in very specialized ways.

Unless you want to script Java or .NET applications with Python, you probably want to use the standard CPython system. Because it is the reference implementation of the language, it tends to run fastest, be the most complete, and be more robust than the alternative systems. Figure 2-2 reflects CPython’s runtime architecture.

The Jython system (originally known as JPython) is an alternative implementation of the Python language, targeted for integration with the Java programming language. Jython consists of Java classes that compile Python source code to Java byte code, and then route the resulting byte code to the Java Virtual Machine ( JVM). Programmers still code Python statements in .py text files as usual; the Jython system essentially just replaces the rightmost two bubbles in Figure 2-2 with Java-based equivalents.

Jython’s goal is to allow Python code to script Java applications, much as CPython allows Python to script C and C++ components. Its integration with Java is remarkably seamless. Because Python code is translated to Java byte code, it looks and feels like a true Java program at runtime. Jython scripts can serve as web applets and servlets, build Java-based GUIs, and so on. Moreover, Jython includes integration support that allows Python code to import and use Java classes, as though they were coded in Python. Because Jython is slower and less robust than CPython, it is usually seen as a tool of interest primarily to Java developers.

A third, and still somewhat experimental implementation of Python, is designed to allow Python programs to integrate with applications coded to Microsoft’s .NET framework. .NET and its C# programming language runtime system are designed to be a language-neutral object communication layer, in the spirit of Microsoft’s earlier COM model. Python.NET allows Python programs to act as both client and server components, accessible from other .NET languages.

By implementation, Python.NET is very much like Jython—it replaces the last two bubbles in Figure 2-2 for execution in the .NET environment. Also like Jython, Python.NET has a special focus—it is primarily of interest to developers integrating Python with .NET components. (Python.NET’s evolution is unclear as we write this; for more details, consult Python online resources.) 

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