I Am Lost at C

So far in this book, weve been using Python as it comes out of the box. We have used interfaces to services outside Python, and coded extensions as Python modules. But we haven added any external services beyond the built-in set. For many users, this makes perfect sense: such standalone programming is one of the main ways people apply Python. As weve seen, Python comes with batteries included -- interfaces to system tools, Internet protocols, GUIs, filesystems, and much more.

But for many systems, Pythons ability to integrate with C-compatible components is a crucial feature of the language. In fact, Pythons role as an extension and interface language in larger systems is one of the reasons for its popularity and why it is often called a "scripting" language. Its design supports hybrid systems that mix components written in a variety of programming languages. Because different languages have different strengths, being able to pick and choose on a component-by-component basis is a powerful concept. You can add Python to the mix anywhere you need an easy-to-use and flexible language tool.

For instance, compiled languages such as C and C++ are optimized for speed of execution, but are complex to program -- for developers, but especially for end users. Because Python is optimized for speed of development, using Python scripts to control or customize software components written in C or C++ can yield more flexible systems and dramatically faster development modes. Systems designed to delegate customizations to Python scripts don need to be shipped with full source code and don require end users to learn complex or proprietary languages. Moreover, moving selected components of a pure Python program to C can optimize program performance.

.1.1 Integration Topics

The last two technical chapters of this book introduce Pythons tools for interfacing to the outside world, and discuss both its ability to be used as an embedded language tool in other systems and its interfaces for extending Python scripts with new modules and types implemented in C-compatible languages. Ill also summarize other integration techniques that are less C-specific, such as COM and JPython.

When you mix Python with C components, either Python or C can be "on top." Because of that, there are two distinct integration APIs:

• The extending interface for running C extensions from Python programs
• The embedding interface for running Python code from C programs

This chapter covers extending, and the next explores embedding. Some systems use only one scheme, but many use both. For instance, embedded Python code run from C can also use linked-in C extensions to interface with the enclosing application. And in callback-based systems, C code accessed through extending interfaces may later use embedding techniques to run Python callback handlers. Python has an open and reentrant architecture that lets you mix languages arbitrarily.

Before we get into details, I should mention that Python/C integration is a big topic -- in principle, the entire set of extern C functions in the Python system makes up its runtime interface. The next two chapters concentrate only on the tools commonly used to implement integration with external modules. For additional examples beyond this book and its CD (view CD-ROM content online at http://examples.oreilly.com/python2), see the Python source code itself; its Modules and Objects directories are a wealth of code resources. Most of the Python built-ins we have used in this book -- from simple things such as integers and strings to more advanced tools such as files, system calls, Tkinter, and DBM -- utilize integration APIs and can be studied in Pythons source code distribution.

These chapters assume that you know basic C programming concepts. If you don , you won miss much by skipping or skimming these chapters. Typically, C developers code the extending and embedding interfaces of a system, and others do the bulk of the systems programming with Python alone. But if you know enough about C programming to recognize a need for an extension language, you probably already have the required background knowledge for this chapter. The good news in both chapters is that much of the complexity inherent in integrating Python with a static compiled language like C can be automated with tools such as SWIG in the extension domain, and higher-level APIs in the embedding world.