4. Using Python on Windows (2024)

4.1.1. Installation steps¶

Four Python 3.12 installers are available for download - two each for the32-bit and 64-bit versions of the interpreter. The web installer is a smallinitial download, and it will automatically download the required components asnecessary. The offline installer includes the components necessary for adefault installation and only requires an internet connection for optionalfeatures. See Installing Without Downloading for other ways to avoid downloadingduring installation.

After starting the installer, one of two options may be selected:

If you select “Install Now”:

• You will not need to be an administrator (unless a system update for theC Runtime Library is required or you install the Python Launcher for Windows for allusers)

• Python will be installed into your user directory

• The Python Launcher for Windows will be installed according to the option at the bottomof the first page

• The standard library, test suite, launcher and pip will be installed

• If selected, the install directory will be added to your PATH

• Shortcuts will only be visible for the current user

Selecting “Customize installation” will allow you to select the features toinstall, the installation location and other options or post-install actions.To install debugging symbols or binaries, you will need to use this option.

To perform an all-users installation, you should select “Customizeinstallation”. In this case:

• You may be required to provide administrative credentials or approval

• Python will be installed into the Program Files directory

• The Python Launcher for Windows will be installed into the Windows directory

• Optional features may be selected during installation

• The standard library can be pre-compiled to bytecode

• If selected, the install directory will be added to the system PATH

• Shortcuts are available for all users

4.1.2. Removing the MAX_PATH Limitation¶

Windows historically has limited path lengths to 260 characters. This meant thatpaths longer than this would not resolve and errors would result.

In the latest versions of Windows, this limitation can be expanded toapproximately 32,000 characters. Your administrator will need to activate the“Enable Win32 long paths” group policy, or set LongPathsEnabled to 1in the registry keyHKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem.

This allows the open() function, the os module and most otherpath functionality to accept and return paths longer than 260 characters.

After changing the above option, no further configuration is required.

Changed in version 3.6: Support for long paths was enabled in Python.

4.1.3. Installing Without UI¶

All of the options available in the installer UI can also be specified from thecommand line, allowing scripted installers to replicate an installation on manymachines without user interaction. These options may also be set withoutsuppressing the UI in order to change some of the defaults.

The following options (found by executing the installer with /?) can bepassed into the installer:

All other options are passed as name=value, where the value is usually0 to disable a feature, 1 to enable a feature, or a path. The full listof available options is shown below.

For example, to silently install a default, system-wide Python installation,you could use the following command (from an elevated command prompt):

python-3.9.0.exe /quiet InstallAllUsers=1 PrependPath=1 Include_test=0

To allow users to easily install a personal copy of Python without the testsuite, you could provide a shortcut with the following command. This willdisplay a simplified initial page and disallow customization:

python-3.9.0.exe InstallAllUsers=0 Include_launcher=0 Include_test=0 SimpleInstall=1 SimpleInstallDescription="Just for me, no test suite."

(Note that omitting the launcher also omits file associations, and is onlyrecommended for per-user installs when there is also a system-wide installationthat included the launcher.)

The options listed above can also be provided in a file named unattend.xmlalongside the executable. This file specifies a list of options and values.When a value is provided as an attribute, it will be converted to a number ifpossible. Values provided as element text are always left as strings. Thisexample file sets the same options as the previous example:

<Options> <Option Name="InstallAllUsers" Value="no" /> <Option Name="Include_launcher" Value="0" /> <Option Name="Include_test" Value="no" /> <Option Name="SimpleInstall" Value="yes" /> <Option Name="SimpleInstallDescription">Just for me, no test suite</Option></Options>

4.1.4. Installing Without Downloading¶

As some features of Python are not included in the initial installer download,selecting those features may require an internet connection. To avoid thisneed, all possible components may be downloaded on-demand to create a completelayout that will no longer require an internet connection regardless of theselected features. Note that this download may be bigger than required, butwhere a large number of installations are going to be performed it is veryuseful to have a locally cached copy.

Execute the following command from Command Prompt to download all possiblerequired files. Remember to substitute python-3.9.0.exe for the actualname of your installer, and to create layouts in their own directories toavoid collisions between files with the same name.

python-3.9.0.exe /layout [optional target directory]

You may also specify the /quiet option to hide the progress display.

4.1.5. Modifying an install¶

Once Python has been installed, you can add or remove features through thePrograms and Features tool that is part of Windows. Select the Python entry andchoose “Uninstall/Change” to open the installer in maintenance mode.

“Modify” allows you to add or remove features by modifying the checkboxes -unchanged checkboxes will not install or remove anything. Some options cannot bechanged in this mode, such as the install directory; to modify these, you willneed to remove and then reinstall Python completely.

“Repair” will verify all the files that should be installed using the currentsettings and replace any that have been removed or modified.

“Uninstall” will remove Python entirely, with the exception of thePython Launcher for Windows, which has its own entry in Programs and Features.

4.2.1. Known issues¶

>>> import os>>> test_file = 'C:\\Users\\example\\AppData\\Local\\test.txt'>>> os.path.realpath(test_file)'C:\\Users\\example\\AppData\\Local\\Packages\\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\\LocalCache\\Local\\test.txt'

4.4.1. Python Application¶

An application written in Python does not necessarily require users to be awareof that fact. The embedded distribution may be used in this case to include aprivate version of Python in an install package. Depending on how transparent itshould be (or conversely, how professional it should appear), there are twooptions.

Using a specialized executable as a launcher requires some coding, but providesthe most transparent experience for users. With a customized launcher, there areno obvious indications that the program is running on Python: icons can becustomized, company and version information can be specified, and fileassociations behave properly. In most cases, a custom launcher should simply beable to call Py_Main with a hard-coded command line.

The simpler approach is to provide a batch file or generated shortcut thatdirectly calls the python.exe or pythonw.exe with the requiredcommand-line arguments. In this case, the application will appear to be Pythonand not its actual name, and users may have trouble distinguishing it from otherrunning Python processes or file associations.

With the latter approach, packages should be installed as directories alongsidethe Python executable to ensure they are available on the path. With thespecialized launcher, packages can be located in other locations as there is anopportunity to specify the search path before launching the application.

4.4.2. Embedding Python¶

Applications written in native code often require some form of scriptinglanguage, and the embedded Python distribution can be used for this purpose. Ingeneral, the majority of the application is in native code, and some part willeither invoke python.exe or directly use python3.dll. For either case,extracting the embedded distribution to a subdirectory of the applicationinstallation is sufficient to provide a loadable Python interpreter.

As with the application use, packages can be installed to any location as thereis an opportunity to specify search paths before initializing the interpreter.Otherwise, there is no fundamental differences between using the embeddeddistribution and a regular installation.

4.6.1. Excursus: Setting environment variables¶

Windows allows environment variables to be configured permanently at both theUser level and the System level, or temporarily in a command prompt.

To temporarily set environment variables, open Command Prompt and use theset command:

C:\>set PATH=C:\Program Files\Python 3.9;%PATH%C:\>set PYTHONPATH=%PYTHONPATH%;C:\My_python_libC:\>python

These changes will apply to any further commands executed in that console, andwill be inherited by any applications started from the console.

Including the variable name within percent signs will expand to the existingvalue, allowing you to add your new value at either the start or the end.Modifying PATH by adding the directory containingpython.exe to the start is a common way to ensure the correct versionof Python is launched.

To permanently modify the default environment variables, click Start and searchfor ‘edit environment variables’, or open System properties, Advancedsystem settings and click the Environment Variables button.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).

4.6.2. Finding the Python executable¶

Changed in version 3.5.

Besides using the automatically created start menu entry for the Pythoninterpreter, you might want to start Python in the command prompt. Theinstaller has an option to set that up for you.

On the first page of the installer, an option labelled “Add Python to PATH”may be selected to have the installer add the install location into thePATH. The location of the Scripts\ folder is also added.This allows you to type python to run the interpreter, andpip for the package installer. Thus, you can also execute yourscripts with command line options, see Command line documentation.

If you don’t enable this option at install time, you can always re-run theinstaller, select Modify, and enable it. Alternatively, you can manuallymodify the PATH using the directions in Excursus: Setting environment variables. Youneed to set your PATH environment variable to include the directoryof your Python installation, delimited by a semicolon from other entries. Anexample variable could look like this (assuming the first two entries alreadyexisted):

C:\WINDOWS\system32;C:\WINDOWS;C:\Program Files\Python 3.9

4.8.1. Getting started¶

py

py -3.7

py -2

'py' is not recognized as an internal or external command,operable program or batch file.

py --list

# Select any '3.*' tagged runtimepy -V:3# Select any 'PythonCore' released runtimepy -V:PythonCore/# Select PythonCore's latest Python 3 runtimepy -V:PythonCore/3

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

py hello.py

#! python3

4.8.2. 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 nativesupport for such lines and they are commonly used on such systems to indicatehow a script should be executed. This launcher allows the same facilities tobe used 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:

• /usr/bin/env

• /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 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.

Any of the above virtual commands can be suffixed with an explicit version(either just the major version, or the major and minor version).Furthermore the 32-bit version can be requested by adding “-32” after theminor version. I.e. /usr/bin/python3.7-32 will request usage of the32-bit python 3.7.

Added in version 3.7: Beginning with python launcher 3.7 it is possible to request 64-bit versionby the “-64” suffix. Furthermore it is possible to specify a major andarchitecture without minor (i.e. /usr/bin/python3-64).

Changed in version 3.11: The “-64” suffix is deprecated, and now implies “any architecture that isnot provably i386/32-bit”. To request a specific environment, use the new-V:TAG argument with the complete tag.

The /usr/bin/env form of shebang line has one further special property.Before looking for installed Python interpreters, this form will search theexecutable PATH for a Python executable matching the name providedas the first argument. This corresponds to the behaviour of the Unix envprogram, which performs a PATH search.If an executable matching the first argument after the env command cannotbe found, but the argument starts with python, it will be handled asdescribed for the other virtual commands.The environment variable PYLAUNCHER_NO_SEARCH_PATH may be set(to any value) to skip this search of PATH.

Shebang lines that do not match any of these patterns are looked up in the[commands] section of the launcher’s .INI file.This may be used to handle certain commands in a way that makes sense for yoursystem. The name of the command must be a single argument (no spaces in theshebang executable), and the value substituted is the full path to theexecutable (additional arguments specified in the .INI will be quoted as partof the filename).

[commands]/bin/xpython=C:\Program Files\XPython\python.exe

Any commands not found in the .INI file are treated as Windows executablepaths that are absolute or relative to the directory containing the script file.This is a convenience for Windows-only scripts, such as those generated by aninstaller, since the behavior is not compatible with Unix-style shells.These paths may be quoted, and may include multiple arguments, after which thepath to the script and any additional arguments will be appended.

4.8.3. Arguments in shebang lines¶

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

4.8.4. Customization¶

[defaults]python=3.7

[defaults]python=3python3=3.7

4.8.5. Diagnostics¶

If an environment variable PYLAUNCHER_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 verbose and 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. It is primarily intended for testing and debugging.

4.8.6. Dry Run¶

If an environment variable PYLAUNCHER_DRYRUN is set (to any value),the launcher will output the command it would have run, but will not actuallylaunch Python. This may be useful for tools that want to use the launcher todetect and then launch Python directly. Note that the command written tostandard output is always encoded using UTF-8, and may not render correctly inthe console.

4.8.7. Install on demand¶

If an environment variable PYLAUNCHER_ALLOW_INSTALL is set (to anyvalue), and the requested Python version is not installed but is available onthe Microsoft Store, the launcher will attempt to install it. This may requireuser interaction to complete, and you may need to run the command again.

An additional PYLAUNCHER_ALWAYS_INSTALL variable causes the launcherto always try to install Python, even if it is detected. This is mainly intendedfor testing (and should be used with PYLAUNCHER_DRYRUN).

4.8.8. Return codes¶

The following exit codes may be returned by the Python launcher. Unfortunately,there is no way to distinguish these from the exit code of Python itself.

The names of codes are as used in the sources, and are only for reference. Thereis no way to access or resolve them apart from reading this page. Entries arelisted in alphabetical order of names.

4.10.1. PyWin32¶

The PyWin32 module by Mark Hammondis a collection of modules for advanced Windows-specific support. This includesutilities for:

• Component Object Model(COM)

• Win32 API calls

• Registry

• Event log

• Microsoft Foundation Classes(MFC) user interfaces

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

4.10.2. cx_Freeze¶

cx_Freezewraps Python scripts into executable Windows programs(*.exe files). When you have done this, you can distribute yourapplication without requiring your users to install Python.