Learning Python - Mark Lutz [55]
Such an embedded execution mode is commonly used to support end-user customization—a game program, for instance, might allow for play modifications by running user-accessible embedded Python code at strategic points in time. Users can modify this type of system by providing or changing Python code. Because Python code is interpreted, there is no need to recompile the entire system to incorporate the change (see Chapter 2 for more on how Python code is run).
In this mode, the enclosing system that runs your code might be written in C, C++, or even Java when the Jython system is used. As an example, it’s possible to create and run strings of Python code from a C program by calling functions in the Python runtime API (a set of services exported by the libraries created when Python is compiled on your machine):
#include ... Py_Initialize(); // This is C, not Python PyRun_SimpleString("x = 'brave ' + 'sir robin'"); // But it runs Python code In this C code snippet, a program coded in the C language embeds the Python interpreter by linking in its libraries, and passes it a Python assignment statement string to run. C programs may also gain access to Python modules and objects and process or execute them using other Python API tools. This book isn’t about Python/C integration, but you should be aware that, depending on how your organization plans to use Python, you may or may not be the one who actually starts the Python programs you create. Regardless, you can usually still use the interactive and file-based launching techniques described here to test code in isolation from those enclosing systems that may eventually use it.[11] Frozen Binary Executables Frozen binary executables, described in Chapter 2, are packages that combine your program’s byte code and the Python interpreter into a single executable program. This approach enables Python programs to be launched in the same ways that you would launch any other executable program (icon clicks, command lines, etc.). While this option works well for delivery of products, it is not really intended for use during program development; you normally freeze just before shipping (after development is finished). See the prior chapter for more on this option. Text Editor Launch Options As mentioned previously, although they’re not full-blown IDE GUIs, most programmer-friendly text editors have support for editing, and possibly running, Python programs. Such support may be built in or fetchable on the Web. For instance, if you are familiar with the Emacs text editor, you can do all your Python editing and launching from inside that text editor. See the text editor resources page at http://www.python.org/editors for more details, or search the Web for the phrase “Python editors.” Still Other Launch Options Depending on your platform, there may be additional ways that you can start Python programs. For instance, on some Macintosh systems you may be able to drag Python program file icons onto the Python interpreter icon to make them execute, and on Windows you can always start Python scripts with the Run... option in the Start menu. Additionally, the Python standard library has utilities that allow Python programs to be started by other Python programs in separate processes (e.g., os.popen, os.system), and Python scripts might also be spawned in larger contexts like the Web (for instance, a web page might invoke a script on a server); however, these are beyond the scope of the present chapter. Future Possibilities? This chapter reflects current practice, but much of the material is both platform- and time-specific. Indeed, many of the execution and launch details presented arose during the shelf life of this book’s various editions. As with program execution options, it’s not impossible that