3.3.1. Excursus: Setting environment variables

Windows has a built-in dialog for changing environment variables (following guide applies to XP classical view): Right-click the icon for your machine (usually located on your Desktop and called “My Computer”) and choose Properties there. Then, open the Advanced tab and click the Environment Variables button.

In short, your path is:

My Computer ‣ Properties ‣ Advanced ‣ Environment Variables

In this dialog, you can add or modify User and System variables. To change System variables, you need non-restricted access to your machine (i.e. Administrator rights).

Another way of adding variables to your environment is using the set command:

set PYTHONPATH=%PYTHONPATH%;C:\My_python_lib

To make this setting permanent, you could add the corresponding command line to your autoexec.bat. msconfig is a graphical interface to this file.

Viewing environment variables can also be done more straight-forward: The command prompt will expand strings wrapped into percent signs automatically:

echo %PATH%

Consult set /? for details on this behaviour.

3.3.2. Finding the Python executable

Changed in version 3.3.

Besides using the automatically created start menu entry for the Python interpreter, you might want to start Python in the command prompt. As of Python 3.3, the installer has an option to set that up for you.

At the “Customize Python 3.3” screen, an option called “Add python.exe to search path” can be enabled to have the installer place your installation into the %PATH%. This allows you to type python to run the interpreter. Thus, you can also execute your scripts with command line options, see Command line documentation.

If you don’t enable this option at install time, you can always re-run the installer to choose it.

The alternative is manually modifying the %PATH% using the directions in Excursus: Setting environment variables. You need to set your %PATH% environment variable to include the directory of your Python distribution, delimited by a semicolon from other entries. An example variable could look like this (assuming the first two entries are Windows’ default):

C:\WINDOWS\system32;C:\WINDOWS;C:\Python33

3.3.3. Finding modules

Python usually stores its library (and thereby your site-packages folder) in the installation directory. So, if you had installed Python to C:\Python\, the default library would reside in C:\Python\Lib\ and third-party modules should be stored in C:\Python\Lib\site-packages\.

This is how sys.path is populated on Windows:

• An empty entry is added at the start, which corresponds to the current directory.
• If the environment variable PYTHONPATH exists, as described in Environment variables, its entries are added next. Note that on Windows, paths in this variable must be separated by semicolons, to distinguish them from the colon used in drive identifiers (C:\ etc.).
• Additional “application paths” can be added in the registry as subkeys of \SOFTWARE\Python\PythonCore\version\PythonPath under both the HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE hives. Subkeys which have semicolon-delimited path strings as their default value will cause each path to be added to sys.path. (Note that all known installers only use HKLM, so HKCU is typically empty.)
• If the environment variable PYTHONHOME is set, it is assumed as “Python Home”. Otherwise, the path of the main Python executable is used to locate a “landmark file” (Lib\os.py) to deduce the “Python Home”. If a Python home is found, the relevant sub-directories added to sys.path (Lib, plat-win, etc) are based on that folder. Otherwise, the core Python path is constructed from the PythonPath stored in the registry.
• If the Python Home cannot be located, no PYTHONPATH is specified in the environment, and no registry entries can be found, a default path with relative entries is used (e.g. .\Lib;.\plat-win, etc).

The end result of all this is:

• When running python.exe, or any other .exe in the main Python directory (either an installed version, or directly from the PCbuild directory), the core path is deduced, and the core paths in the registry are ignored. Other “application paths” in the registry are always read.
• When Python is hosted in another .exe (different directory, embedded via COM, etc), the “Python Home” will not be deduced, so the core path from the registry is used. Other “application paths” in the registry are always read.
• If Python can’t find its home and there is no registry (eg, frozen .exe, some very strange installation setup) you get a path with some default, but relative, paths.

3.3.4. Executing scripts

As of Python 3.3, Python includes a launcher which facilitates running Python scripts. See Python Launcher for Windows for more information.

3.3.5. Executing scripts without the Python launcher

Without the Python launcher installed, Python scripts (files with the extension .py) will be executed by python.exe by default. This executable opens a terminal, which stays open even if the program uses a GUI. If you do not want this to happen, use the extension .pyw which will cause the script to be executed by pythonw.exe by default (both executables are located in the top-level of your Python installation directory). This suppresses the terminal window on startup.

You can also make all .py scripts execute with pythonw.exe, setting this through the usual facilities, for example (might require administrative rights):

1. Launch a command prompt.

2. Associate the correct file group with .py scripts:

   assoc .py=Python.File
   

3. Redirect all Python files to the new executable:

   ftype Python.File=C:\Path\to\pythonw.exe "%1" %*
   

3.4.1. Getting started

py

py -2.6

py -3

#! python
import sys
sys.stdout.write("hello from Python %s\n" % (sys.version,))

py hello.py

#! python3

3.4.2. Shebang Lines

If the first line of a script file starts with #!, it is known as a “shebang” line. Linux and other Unix like operating systems have native support for such lines and are commonly used on such systems to indicate how a script should be executed. This launcher allows the same facilities to be using with Python scripts on Windows and the examples above demonstrate their use.

To allow shebang lines in Python scripts to be portable between Unix and Windows, this launcher supports a number of ‘virtual’ commands to specify which interpreter to use. The supported virtual commands are:

• /usr/bin/env python
• /usr/bin/python
• /usr/local/bin/python
• python

For example, if the first line of your script starts with

#! /usr/bin/python

The default Python will be located and used. As many Python scripts written to work on Unix will already have this line, you should find these scripts can be used by the launcher without modification. If you are writing a new script on Windows which you hope will be useful on Unix, you should use one of the shebang lines starting with /usr.

3.4.3. Arguments in shebang lines

The shebang lines can also specify additional options to be passed to the Python interpreter. For example, if you have a shebang line:

#! /usr/bin/python -v

Then Python will be started with the -v option

3.4.4. Customization

[defaults]
python=3.1

[defaults]
python=3
python3=3.1

3.4.5. Diagnostics

If an environment variable PYLAUNCH_DEBUG is set (to any value), the launcher will print diagnostic information to stderr (i.e. to the console). While this information manages to be simultaneously verbose and terse, it should allow you to see what versions of Python were located, why a particular version was chosen and the exact command-line used to execute the target Python.

3.5.1. PyWin32

The PyWin32 module by Mark Hammond is a collection of modules for advanced Windows-specific support. This includes utilities for:

• Component Object Model (COM)
• Win32 API calls
• Registry
• Event log
• Microsoft Foundation Classes (MFC) user interfaces

PythonWin is a sample MFC application shipped with PyWin32. It is an embeddable IDE with a built-in debugger.

3.5.2. Py2exe

Py2exe is a distutils extension (see Extending Distutils) which wraps Python scripts into executable Windows programs (*.exe files). When you have done this, you can distribute your application without requiring your users to install Python.

3.5.3. WConio

Since Python’s advanced terminal handling layer, curses, is restricted to Unix-like systems, there is a library exclusive to Windows as well: Windows Console I/O for Python.

WConio is a wrapper for Turbo-C’s CONIO.H, used to create text user interfaces.