Is Python a

The Python 2.5 MSI Installer on Windows VistaAs I write this in mid-2007, the Python self-installer for Windows is a .msi installationfile. This format works fine on Windows XP (simply double-click on the file, and itruns), but it can have issues on the current version of Windows Vista. In particular,running the MSI installer by clicking on it caused Python to be installed at the root ofthe C: drive on my machine, instead of in the correct C:\Python25 directory. Pythonstill works in the root directory, but this is not the correct place to install it.This is a Vista security-related issue; in short, MSI files are not true executables, so theydo not correctly inherit administrator permissions, even if run by the administratoruser. Instead, MSI files are run via the Widows Registry—their filenames are associatedwith the MSI installer program.To install Python 2.5.1 on my Vista-based OQO handheld, I had to use a commandlineapproach to force the required administrator permissions. Here’s the workaround:go to your Start button, select the All Programs entry, choose Accessories, right-clickon the Command Prompt entry there, choose “Run as administrator,” and select Continuein the access control dialog. Now, within the Command Prompt window, issuea cd command to change to the directory where your Python MSI installer file resides(e.g., cd C:\user\downloads), and then run the MSI installer manually by typing acommand line of the form msiexec /i python-2.5.1.msi. Finally, follow the usual GUIinteractions to complete the install.Naturally, this behavior may change over time. This procedure may not be required infuture versions of Vista, and additional workarounds may be possible (such as disablingVista security, if you dare). It’s also possible that the Python self-installer mayeventually be provided in a different format that fixes this problem in the future—as atrue executable, for instance. Be sure to try your installer by simply clicking its iconbefore attempting any workarounds to see if it works properly.Python Environment VariablesEnvironment variables—known to some as shell variables, or DOS variables—liveoutside Python, and thus can be used to customize the interpreter’s behavior eachtime it is run on a given computer. Python recognizes a handful of environment variablesettings, but only a few are used often enough to warrant explanation here.Table A-1 summarizes the main Python-related environment variable settings.Table A-1. Important environment variablesVariablePATH (or path)PYTHONPATHPYTHONSTARTUPTCL_LIBRARY, TK_LIBRARYRoleSystem shell search path (for finding “python”)Python module search path (for imports)Path to Python interactive startup fileGUI extension variables (Tkinter)642 | Appendix A: Installation and Configuration