Interaction. Using a system command line, IDLE, or other, start the Python interactive command line (>>> prompt), and type the expression: "Hello World!" (including the quotes). The string should be echoed back to you. The purpose of this exercise is to get your environment configured to run Python. In some scenarios, you may need to first run a cd shell command, type the full path to the python executable, or add its path to your PATH environment variable. If desired, you can set it in your .cshrc or .kshrc file to make Python permanently available on Unix systems; on Windows use a setup.bat, autoexec.bat, or the environment variable GUI. See Appendix A for help with environment variable settings.

Programs. With the text editor of your choice, write a simple module file—a file containing the single statement: print 'Hello module world!'. Store this statement in a file named module1.py. Now, run this file by using any launch option you like: running it in IDLE, clicking on its file icon, passing it to the Python interpreter program on the system shell's command line, and so on. In fact, experiment by running your file with as many of the launch techniques seen in this chapter as you can. Which technique seems easiest? (There is no right answer to this one.)

Modules. Next, start the Python interactive command line (>>> prompt) and import the module you wrote in Exercise 2. Try moving the file to a different directory and importing it again from its original directory (i.e., run Python in the original directory when you import); what happens? (Hint: is there still a file named module1.pyc in the original directory?)

Scripts. If your platform supports it, add the #! line to the top of your module1.py module, give the file executable privileges, and run it directly as an executable. What does the first line need to contain? Skip that if you are working on a Windows machine (#! usually only has meaning on Unix and Linux); instead try running your file by listing just its name in a DOS console window (this works on recent versions of Windows), or the Start/Run... dialog box.

Errors. Experiment with typing mathematical expressions and assignments at the Python interactive command line. First type the expression: 1 / 0; what happens? Next, type a variable name you haven't assigned a value to yet; what happens this time?

You may not know it yet, but you're doing exception processing, a topic we'll explore in depth in Part VII. As you'll learn there, you are technically triggering what's known as the default exception handler—logic that prints a standard error message.

For full-blown source code debugging chores, IDLE includes a GUI debugging interface introduced in this chapter (see the advanced IDLE usage section), and a Python standard library module named pdb provides a command-line debugging interface (more on pdb in the standard library manual). When first starting out, Python's default error messages will probably be as much error handling as you need—they give the cause of the error, as well as showing the lines in your code that were active when the error occurred.

Breaks. At the Python command line, type:

L = [1, 2]
L.append(L)
L

What happens? If you're using a Python newer than Release 1.5, you'll probably see a strange output that we'll describe in the next part of the book. If you're using a Python version older than 1.5.1, a Ctrl-C key combination will probably help on most platforms. Why do you think this occurs? What does Python report when you type the Ctrl-C key combination? Warning: if you do have a Python older than Release 1.5.1, make sure your machine can stop a program with a break-key combination of some sort before running this test, or you may be waiting a long time.

Documentation. Spend at least 17 minutes browsing the Python library and language manuals before moving on, to get a feel for the available tools in the standard library, and the structure of the documentation set. It takes at least this long to become familiar with the location of major topics in the manual set; once you do, it's easy to find what you need. You can find this manual in the Python Start button entry on Windows, in the Help pulldown menu in IDLE, or online at http://www.python.org/doc. We'll also have a few more words to say about the manuals and other documentation sources available (including PyDoc and the help function), in Chapter 11. If you still have time, go explore the Python web site (http://www.python.org), and the Vaults of Parnassus site link you'll find there. Especially check out the python.org documentation and search pages; they can be crucial resources in practice.