This document aims to give an overview of Windows-specific behaviour you shouldknow about when using Python on Microsoft Windows.
Unlike most Unix systems and services, Windows does not require Python nativelyand thus does not pre-install a version of Python. However, the CPython teamhas compiled Windows installers (MSI packages) with everyrelease for many years.
With ongoing development of Python, some platforms that used to be supportedearlier are no longer supported (due to the lack of users or developers).CheckPEP 11 for details on all unsupported platforms.
SeePython for Windowsfor detailed information about platforms with pre-compiled installers.
See also
Besides the standard CPython distribution, there are modified packages includingadditional functionality. The following is a list of popular versions and theirkey features:
Notice that these packages are likely to installolder versions of Python.
In order to run Python flawlessly, you might have to change certain environmentsettings in Windows.
Windows has a built-in dialog for changing environment variables (followingguide applies to XP classical view): Right-click the icon for your machine(usually located on your Desktop and called “My Computer”) and chooseProperties there. Then, open theAdvanced taband click theEnvironment 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 changeSystem variables, you need non-restricted access to your machine(i.e. Administrator rights).
Another way of adding variables to your environment is using thesetcommand:
set PYTHONPATH=%PYTHONPATH%;C:\My_python_lib
To make this setting permanent, you could add the corresponding command line toyourautoexec.bat.msconfig is a graphical interface to thisfile.
Viewing environment variables can also be done more straight-forward: Thecommand prompt will expand strings wrapped into percent signs automatically:
echo %PATH%
Consultset /? for details on this behaviour.
See also
Changed in version 3.3.
Besides using the automatically created start menu entry for the Pythoninterpreter, you might want to start Python in the command prompt. As ofPython 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 placeyour installation into the%PATH%. This allows you to typepython to run the interpreter. Thus, you can also execute yourscripts with command line options, seeCommand line documentation.
If you don’t enable this option at install time, you can always re-run theinstaller to choose it.
The alternative is manually modifying the%PATH% using thedirections inExcursus: 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 looklike this (assuming the first two entries are Windows’ default):
C:\WINDOWS\system32;C:\WINDOWS;C:\Python33
Python usually stores its library (and thereby your site-packages folder) in theinstallation directory. So, if you had installed Python toC:\Python\, the default library would reside inC:\Python\Lib\ and third-party modules should be stored inC:\Python\Lib\site-packages\.
This is howsys.path is populated on Windows:
The end result of all this is:
As of Python 3.3, Python includes a launcher which facilitates running Pythonscripts. SeePython Launcher for Windows for more information.
Without the Python launcher installed, Python scripts (files with the extension.py) will be executed bypython.exe by default. This executableopens a terminal, which stays open even if the program uses a GUI. If you donot want this to happen, use the extension.pyw which will cause the scriptto be executed bypythonw.exe by default (both executables arelocated in the top-level of your Python installation directory). Thissuppresses the terminal window on startup.
You can also make all.py scripts execute withpythonw.exe,setting this through the usual facilities, for example (might requireadministrative rights):
Launch a command prompt.
Associate the correct file group with.py scripts:
assoc .py=Python.File
Redirect all Python files to the new executable:
ftype Python.File=C:\Path\to\pythonw.exe "%1" %*
New in version 3.3.
The Python launcher for Windows is a utility which aids in the location andexecution of different Python versions. It allows scripts (or thecommand-line) to indicate a preference for a specific Python version, andwill locate and execute that version.
You should ensure the launcher is on your PATH - depending on how it wasinstalled it may already be there, but check just in case it is not.
From a command-prompt, execute the following command:
py
You should find that the latest version of Python 2.x you have installed isstarted - it can be exited as normal, and any additional command-linearguments specified will be sent directly to Python.
If you have multiple versions of Python 2.x installed (e.g., 2.6 and 2.7) youwill have noticed that Python 2.7 was started - to launch Python 2.6, try thecommand:
py -2.6
If you have a Python 3.x installed, try the command:
py -3
You should find the latest version of Python 3.x starts.
Let’s create a test Python script - create a file calledhello.py with thefollowing contents
#! pythonimport syssys.stdout.write("hello from Python %s\n" % (sys.version,))From the directory in which hello.py lives, execute the command:
py hello.py
You should notice the version number of your latest Python 2.x installationis printed. Now try changing the first line to be:
#! python3
Re-executing the command should now print the latest Python 3.x information.As with the above command-line examples, you can specify a more explicitversion qualifier. Assuming you have Python 2.6 installed, try changing thefirst line to#!python2.6 and you should find the 2.6 versioninformation printed.
The launcher should have been associated with Python files (i.e..py,.pyw,.pyc,.pyo files) when it was installed. This means thatwhen you double-click on one of these files from Windows explorer the launcherwill be used, and therefore you can use the same facilities described above tohave the script specify the version which should be used.
The key benefit of this is that a single launcher can support multiple Pythonversions at the same time depending on the contents of the first line.
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 nativesupport for such lines and are commonly used on such systems to indicate howa script should be executed. This launcher allows the same facilities to beusing with Python scripts on Windows and the examples above demonstrate theiruse.
To allow shebang lines in Python scripts to be portable between Unix andWindows, this launcher supports a number of ‘virtual’ commands to specifywhich interpreter to use. The supported virtual commands are:
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 writtento work on Unix will already have this line, you should find these scripts canbe used by the launcher without modification. If you are writing a new scripton Windows which you hope will be useful on Unix, you should use one of theshebang lines starting with/usr.
The shebang lines can also specify additional options to be passed to thePython interpreter. For example, if you have a shebang line:
#! /usr/bin/python -v
Then Python will be started with the-v option
Two .ini files will be searched by the launcher -py.ini in thecurrent user’s “application data” directory (i.e. the directory returnedby calling the Windows function SHGetFolderPath with CSIDL_LOCAL_APPDATA)andpy.ini in the same directory as the launcher. The same .inifiles are used for both the ‘console’ version of the launcher (i.e.py.exe) and for the ‘windows’ version (i.e. pyw.exe)
Customization specified in the “application directory” will haveprecedence over the one next to the executable, so a user, who may nothave write access to the .ini file next to the launcher, can overridecommands in that global .ini file)
In some cases, a version qualifier can be included in a command to dictatewhich version of Python will be used by the command. A version qualifierstarts with a major version number and can optionally be followed by a period(‘.’) and a minor version specifier. If the minor qualifier is specified, itmay optionally be followed by “-32” to indicate the 32-bit implementation ofthat version be used.
For example, a shebang line of#!python has no version qualifier, while#!python3 has a version qualifier which specifies only a major version.
If no version qualifiers are found in a command, the environment variablePY_PYTHON can be set to specify the default version qualifier - the defaultvalue is “2”. Note this value could specify just a major version (e.g. “2”) ora major.minor qualifier (e.g. “2.6”), or even major.minor-32.
If no minor version qualifiers are found, the environment variablePY_PYTHON{major} (where{major} is the current major version qualifieras determined above) can be set to specify the full version. If no such optionis found, the launcher will enumerate the installed Python versions and usethe latest minor release found for the major version, which is likely,although not guaranteed, to be the most recently installed version in thatfamily.
On 64-bit Windows with both 32-bit and 64-bit implementations of the same(major.minor) Python version installed, the 64-bit version will always bepreferred. This will be true for both 32-bit and 64-bit implementations of thelauncher - a 32-bit launcher will prefer to execute a 64-bit Python installationof the specified version if available. This is so the behavior of the launchercan be predicted knowing only what versions are installed on the PC andwithout regard to the order in which they were installed (i.e., without knowingwhether a 32 or 64-bit version of Python and corresponding launcher wasinstalled last). As noted above, an optional “-32” suffix can be used on aversion specifier to change this behaviour.
Examples:
In addition to environment variables, the same settings can be configuredin the .INI file used by the launcher. The section in the INI file iscalled[defaults] and the key name will be the same as theenvironment variables without the leadingPY_ prefix (and note thatthe key names in the INI file are case insensitive.) The contents ofan environment variable will override things specified in the INI file.
For example:
[defaults]python=3.1
[defaults]python=3python3=3.1
If an environment variablePYLAUNCH_DEBUG is set (to any value), thelauncher will print diagnostic information to stderr (i.e. to the console).While this information manages to be simultaneously verboseand terse, itshould allow you to see what versions of Python were located, why aparticular version was chosen and the exact command-line used to execute thetarget Python.
Even though Python aims to be portable among all platforms, there are featuresthat are unique to Windows. A couple of modules, both in the standard libraryand external, and snippets exist to use these features.
The Windows-specific standard modules are documented inMS Windows Specific Services.
ThePyWin32 module by Mark Hammondis a collection of modules for advanced Windows-specific support. This includesutilities for:
PythonWin is a sample MFC applicationshipped with PyWin32. It is an embeddable IDE with a built-in debugger.
See also
cx_Freeze is adistutilsextension (seeExtending Distutils) which wraps Python scripts intoexecutable Windows programs (*.exe files). When you have done this,you can distribute your application without requiring your users to installPython.
If you want to compile CPython yourself, first thing you should do is get thesource. You can download either thelatest release’s source or just grab a freshcheckout.
The source tree contains a build solution and project files for MicrosoftVisual C++, which is the compiler used to build the official Python releases.View thereadme.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 |
| PC/VS9.0/ | 9.0 | 2008 |
| PCbuild/ | 10.0 | 2010 |
Note that any build directories within thePC directory are notnecessarily fully supported. ThePCbuild directory contains the filesfor the compiler used to build the official release.
CheckPCbuild/readme.txt for general information on the build process.
For extension modules, consultBuilding C and C++ Extensions on Windows.
See also
See also
2. Using Python on Unix platforms
4. Using Python on a Macintosh
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