At this point, we’ve seen how to run code typed interactively, and how to launch code saved in files in a variety of ways—system command lines, icon clicks, imports and execs, GUIs like IDLE, and more. That covers most of the techniques in common use, and enough to run the code you’ll see in this book. There are additional ways to run Python code, though, most of which have special or narrow roles. For completeness and reference, the next few sections take a quick look at some of these.
In some specialized domains, Python code may be run automatically by an enclosing system. In such cases, we say that the Python programs are embedded in (i.e., run by) another program. The Python code itself may be entered into a text file, stored in a database, fetched from an HTML page, parsed from an XML document, and so on. But from an operational perspective, another system—not you—may tell Python to run the code you’ve created.
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 <Python.h>
...
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.[10]
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.
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.”
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 some Windows systems 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.
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 new program launch options may arise over time.
New operating systems, and new versions of existing systems, may also provide execution techniques beyond those outlined here. In general, because Python keeps pace with such changes, you should be able to launch Python programs in whatever way makes sense for the machines you use, both now and in the future—be that by swiping on tablet PCs and smartphones, grabbing icons in a virtual reality, or shouting a script’s name over your coworkers’ conversations.
Implementation changes may also impact launch schemes somewhat (e.g., a full compiler could produce normal executables that are launched much like frozen binaries today). If I knew what the future truly held, though, I would probably be talking to a stockbroker instead of writing these words!
[10] See Programming Python (O’Reilly) for more details on embedding Python in C/C++. The embedding API can call Python functions directly, load modules, and more. Also, note that the Jython system allows Java programs to invoke Python code using a Java-based API (a Python interpreter class).