Learning Python - Mark Lutz [37]
How are CPython, Jython, and IronPython different?
Test Your Knowledge: Answers
The Python interpreter is a program that runs the Python programs you write.
Source code is the statements you write for your program—it consists of text in text files that normally end with a .py extension.
Byte code is the lower-level form of your program after Python compiles it. Python automatically stores byte code in files with a .pyc extension.
The PVM is the Python Virtual Machine—the runtime engine of Python that interprets your compiled byte code.
Psyco, Shedskin, and frozen binaries are all variations on the execution model.
CPython is the standard implementation of the language. Jython and IronPython implement Python programs for use in Java and .NET environments, respectively; they are alternative compilers for Python.
Chapter 3. How You Run Programs
OK, it’s time to start running some code. Now that you have a handle on program execution, you’re finally ready to start some real Python programming. At this point, I’ll assume that you have Python installed on your computer; if not, see the prior chapter and Appendix A for installation and configuration hints.
There are a variety of ways to tell Python to execute the code you type. This chapter discusses all the program launching techniques in common use today. Along the way, you’ll learn how to type code interactively and how to save it in files to be run with system command lines, icon clicks, module imports and reloads, exec calls, menu options in GUIs such as IDLE, and more.
If you just want to find out how to run a Python program quickly, you may be tempted to read the parts of this chapter that pertain only to your platform and move on to Chapter 4. But don’t skip the material on module imports, as that’s essential to understanding Python’s program architecture. I also encourage you to at least skim the sections on IDLE and other IDEs, so you’ll know what tools are available for when you start developing more sophisticated Python programs.
The Interactive Prompt
Perhaps the simplest way to run Python programs is to type them at Python’s interactive command line, sometimes called the interactive prompt. There are a variety of ways to start this command line: in an IDE, from a system console, and so on. Assuming the interpreter is installed as an executable program on your system, the most platform-neutral way to start an interactive interpreter session is usually just to type python at your operating system’s prompt, without any arguments. For example:
% python
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ...
Type "help", "copyright", "credits" or "license" for more information.
>>>
Typing the word “python” at your system shell prompt like this begins an interactive Python session; the “%” character at the start of this listing stands for a generic system prompt in this book—it’s not input that you type yourself. The notion of a system shell prompt is generic, but exactly how you access it varies by platform:
On Windows, you can type python in a DOS console window (a.k.a. the Command Prompt, usually found in the Accessories section of the Start→Programs menu) or in the Start→Run... dialog box.
On Unix, Linux, and Mac OS X, you might type this command in a shell or terminal window (e.g., in an xterm or console running a shell such as ksh or csh).
Other systems may use similar or platform-specific devices. On handheld devices, for example, you generally click the Python icon in the home or application window to launch an interactive session.
If you have not set your shell’s PATH environment variable to include Python’s install directory, you may need to replace the word “python” with the full path to the Python executable on your machine. On Unix, Linux, and similar, /usr/local/bin/python or /usr/bin/python will often suffice. On Windows, try typing C:\Python30\python (for version 3.0):
C:\misc> c:\python30\python
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32