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.
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:
- 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:
To make this setting permanent, you could add the corresponding command line to
autoexec.bat. msconfig is a graphical interface to this
Viewing environment variables can also be done more straight-forward: The command prompt will expand strings wrapped into percent signs automatically:
Consult set /? for details on this behaviour.
- Environment variables in Windows NT
- How To Manage Environment Variables in Windows XP
- 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):
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
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
PYTHONPATHexists, 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 (
- Additional “application paths” can be added in the registry as subkeys of
\SOFTWARE\Python\PythonCore\version\PythonPathunder both the
HKEY_LOCAL_MACHINEhives. 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
PYTHONHOMEis 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
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
PYTHONPATHis specified in the environment, and no registry entries can be found, a default path with relative entries is used (e.g.
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
.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
You can also make all
.py scripts execute with pythonw.exe,
setting this through the usual facilities, for example (might require
Launch a command prompt.
Associate the correct file group with
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.
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
- 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.
Py2exe is a
distutils extension (see
Extending Distutils) which wraps Python scripts into executable Windows
*.exe files). When you have done this, you can distribute
your application without requiring your users to install Python.
3.5. Compiling Python on Windows
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|
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.
PC/readme.txt for general information on the build process.
For extension modules, consult Building C and C++ Extensions on Windows.
- 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