3. Using Python on Windows
**************************

This document aims to give an overview of Windows-specific behaviour
you should know about when using Python on Microsoft Windows.


3.1. Installing Python
======================

Unlike most Unix systems and services, Windows does not require Python
natively and thus does not pre-install a version of Python.  However,
the CPython team has compiled Windows installers (MSI packages) with
every release for many years.

With ongoing development of Python, some platforms that used to be
supported earlier are no longer supported (due to the lack of users or
developers). Check **PEP 11** for details on all unsupported
platforms.

* DOS and Windows 3.x are deprecated since Python 2.0 and code
  specific to these systems was removed in Python 2.1.

* Up to 2.5, Python was still compatible with Windows 95, 98 and ME
  (but already raised a deprecation warning on installation).  For
  Python 2.6 (and all following releases), this support was dropped
  and new releases are just expected to work on the Windows NT family.

* Windows CE is still supported.

* The Cygwin installer offers to install the Python interpreter as
  well (cf. Cygwin package source, Maintainer releases)

See Python for Windows (and DOS) for detailed information about
platforms with precompiled installers.

See also:

  Python on XP
     “7 Minutes to “Hello World!”” by Richard Dooling, 2006

  Installing on Windows
     in “Dive into Python: Python from novice to pro” by Mark Pilgrim,
     2004, ISBN 1-59059-356-1

  For Windows users
     in “Installing Python” in “A Byte of Python” by Swaroop C H, 2003


3.2. Alternative bundles
========================

Besides the standard CPython distribution, there are modified packages
including additional functionality.  The following is a list of
popular versions and their key features:

ActivePython
   Installer with multi-platform compatibility, documentation, PyWin32

Enthought Python Distribution
   Popular modules (such as PyWin32) with their respective
   documentation, tool suite for building extensible Python
   applications

Notice that these packages are likely to install *older* versions of
Python.


3.3. Configuring Python
=======================

In order to run Python flawlessly, you might have to change certain
environment settings in Windows.


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.

See also:

  https://support.microsoft.com/kb/100843
     Environment variables in Windows NT

  https://support.microsoft.com/kb/310519
     How To Manage Environment Variables in Windows XP

  https://www.chem.gla.ac.uk/~louis/software/faq/q1.html
     Setting Environment variables, Louis J. Farrugia


3.3.2. Finding the Python executable
------------------------------------

Besides using the automatically created start menu entry for the
Python interpreter, you might want to start Python in the DOS prompt.
To make this work, 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:\Python25

Typing **python** on your command prompt will now fire up the Python
interpreter.  Thus, you can also execute your scripts with command
line options, see Command line documentation.


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
------------------------

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. Additional modules
=======================

Even though Python aims to be portable among all platforms, there are
features that are unique to Windows.  A couple of modules, both in the
standard library and external, and snippets exist to use these
features.

The Windows-specific standard modules are documented in MS Windows
Specific Services.


3.4.1. 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.

See also:

  Win32 How Do I…?
     by Tim Golden

  Python and COM
     by David and Paul Boddie


3.4.2. 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.


3.4.3. 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.


3.5. Compiling Python on Windows
================================

If you want to compile CPython yourself, first thing you should do is
get the source. You can download either the latest release’s source or
just grab a fresh checkout.

For Microsoft Visual C++, which is the compiler with which official
Python releases are built, the source tree contains solutions/project
files.  View the "readme.txt" in their respective directories:

+----------------------+----------------+-------------------------+
| Directory            | MSVC version   | Visual Studio version   |
+======================+================+=========================+
| "PC/VC6/"            | 6.0            | 97                      |
+----------------------+----------------+-------------------------+
| "PC/VS7.1/"          | 7.1            | 2003                    |
+----------------------+----------------+-------------------------+
| "PC/VS8.0/"          | 8.0            | 2005                    |
+----------------------+----------------+-------------------------+
| "PCbuild/"           | 9.0            | 2008                    |
+----------------------+----------------+-------------------------+

Note that not all of these build directories are fully supported.
Read the release notes to see which compiler version the official
releases for your version are built with.

Check "PC/readme.txt" for general information on the build process.

For extension modules, consult Building C and C++ Extensions on
Windows.

See also:

  Python + Windows + distutils + SWIG + gcc MinGW
     or “Creating Python extensions in C/C++ with SWIG and compiling
     them with MinGW gcc under Windows” or “Installing Python
     extension with distutils and without Microsoft Visual C++” by
     Sébastien Sauvage, 2003

  MingW – Python extensions
     by Trent Apted et al, 2007


3.6. Other resources
====================

See also:

  Python Programming On Win32
     “Help for Windows Programmers” by Mark Hammond and Andy Robinson,
     O’Reilly Media, 2000, ISBN 1-56592-621-8

  A Python for Windows Tutorial
     by Amanda Birmingham, 2004
