4. Using Python on Windows

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

Unlike most Unix systems and services, Windows does not include a system
supported installation of Python. To make Python available, the CPython team
has compiled Windows installers with every release for many years. These installers
are primarily intended toadd a per-user installation of Python, with the
core interpreter and library being used by a single user. Theinstaller is also
able toinstall for all users of a single machine, and a separate ZIP file is
available for application-local distributions.

As specify inPEP 11 , a Python release is supports only support a Windows platform
while Microsoft consider the platform under extended support . This is means mean that
Python 3.13 support Windows 8.1 and new . Ifyou require Windows 7
support , please install Python 3.8 .

There are a number of different installers available for Windows, each with
certain benefits and downsides.

Thefull installer contains all components and is the best option for
developers using Python for any kind of project.

TheMicrosoft Store package is a simple installation of Python that is suitable for
running scripts and packages, and using IDLE orother development environments.
It requires Windows 10 and above, but can be safely installed without corrupting other
programs. It also provides many convenient commands for launching Python and
its tools.

Thenuget.org packages are lightweight installations intended for continuous
integration systems. It can be used tobuild Python packages orrun scripts,
but is not updateable and has no user interface tools.

Theembeddable package is a minimal package of Python suitable for
embedding into a larger application.

4.1 .Thefull installer

4.1.1 .installation step

Four Python 3.13 installers are available for download – two each for the
32-bit and 64-bit versions of the interpreter. Theweb installer is a small
initial download, and it will automatically download the required components as
necessary. Theoffline installer includes the components necessary for a
default installation and only requires an internet connection for optional
features. SeeInstalling Without Downloading for other ways toavoid downloading
during installation.

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

Ifyou select “installNow”:

• You will not need tobe an administrator (unless a system update for the
  C Runtime Library is required oryou install the Python Launcher for Windows for all
  users)

• Python will be instal into your user directory

• ThePython Launcher for Windows will be instal accord tothe option at the bottom
  of the first page

• Thestandard library, test suite, launcher and pip will be installed

• Ifselected, the install directory will be added toyour path

• shortcut will only be visible for the current user

select “ customize installation ” will allow you toselect the feature to
install , the installation location and other option orpost – install action .
To install debugging symbol orbinary , you is need will need touse this option .

To perform an all – user installation , you is select should select “ customize
installation ” . In this case :

• You may be required toprovide administrative credentials orapproval

• Python will be instal into the Program Files directory

• ThePython Launcher for Windows will be installed into the Windows directory

• Optional features may be selected during installation

• Thestandard library can be pre-compiled tobytecode

• Ifselected, the install directory will be added tothe system path

• shortcut are available for all users

4.1.2. remove the MAX_path Limitation

Windows historically has limit path length to260 character . This is meant mean that
path long than this would not resolve and error would result .

In the late version of Windows , this limitation can be expand to
approximately 32,000 character . Your administrator is need will need toactivate the
“ enable Win32 long path ” group policy , orsetLongPathsEnabled to1
in the registry key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem.

This allows the open ( ) function, the os module and most other
path functionality toaccept and return paths longer than 260 characters.

After change the above option , no further configuration is require .

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

4.1.3. instal Without UI

All of the options available in the installer UI can also be specified from the
command line, allowing scripted installers toreplicate an installation on many
machines without user interaction. These options may also be set without
suppressing the UI in order tochange some of the defaults.

Thefollowing options (found by executing the installer with /?) can be
passed into the installer:

Name	description
/passive	to display progress without requiring user interaction
/quiet	to install / uninstall without display any UI
/simple	to prevent user customization
/uninstall	to remove Python ( without confirmation )
/layout [ directory ]	to pre – download all component
/log [filename]	to specify log files location

All other options are passed as name=value, where the value is is is usually
0 todisable a feature ,1 toenable a feature, ora path. Thefull list
of available options is shown below.

Name	description	Default
InstallAllUsers	Perform a system-wide installation.	0
targetdir	Theinstallation directory	Selected based on InstallAllUsers
DefaultAllUserstargetdir	Thedefault installation directory for all-user installs	% programfiles%\python X.Y or%ProgramFiles(x86)%\Python X.Y
DefaultJustForMetargetdir	Thedefault install directory for just-for-me installs	%LocalAppData%\Programs\Python\PythonXY or %LocalAppData%\Programs\Python\PythonXY-32 or %LocalAppData%\Programs\Python\PythonXY-64
DefaultCustomtargetdir	Thedefault custom install directory displayed in the UI	( empty )
AssociateFiles	Create file associations if the launcher is also installed.	1
compileall	compile all.py file to .pyc.	0
PrependPath	Prepend install and Scripts directories topath and add .PY topathEXT	0
AppendPath	append install and Scripts directory topath and add .PY topathEXT	0
shortcut	Create shortcuts for the interpreter, documentation and IDLE if installed.	1
Include_doc	installPython manual	1
Include_debug	installdebug binaries	0
Include_dev	install developer header and library . omit this may lead to an unusable installation .	1
Include_exe	installpython.exe and related files. Omitting this may lead toan unusable installation.	1
include_launcher	installPython Launcher for Windows.	1
installlauncheralluser	Installs the launcher for all users. Also requires include_launcher tobe set to1	1
Include_lib	installstandard library and extension modules. Omitting this may lead toan unusable installation.	1
Include_pip	installbundled pip and setuptools	1
Include_symbols	installdebugging symbols (*.pdb)	0
Include_tcltk	installTcl/Tk support and IDLE	1
Include_test	installstandard library test suite	1
Include_tools	installutility scripts	1
LauncherOnly	Only install the launcher . This is override will override most other option .	0
SimpleInstall	disable most install UI	0
SimpleInstalldescription	Acustom message todisplay when the simplified install UI is used.	( empty )

For example, tosilently 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 toeasily install a personal copy of Python without the test
suite, you could provide a shortcut with the following command. This will
display 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 omit the launcher also omit file association , and is only
recommend for per – user install when there is also a system – wide installation
that include the launcher . )

Theoptions listed above can also be provided in a file named unattend.xml
alongside the executable. This file specifies a list of options and values.
When a value is provided as an attribute, it will be converted toa number if
possible. Values provided as element text are always left as strings. This
example 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 this
need, all possible components may be downloaded on-demand tocreate a complete
layout that will no longer require an internet connection regardless of the
selected features. note that this download may be bigger than required, but
where a large number of installations are going tobe performed it is very
useful tohave a locally cached copy.

Execute the following command from Command Prompt todownload all possible
required files. Remember tosubstitute python-3.9.0.exe for the actual
name of your installer, and tocreate layouts in their own directories to
avoid collisions between files with the same name.

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

You may also specify the /quiet option tohide the progress display.

4.1.5 .Modifying an install

Once Python has been installed, you can add orremove features through the
Programs and Features tool that is part of Windows. Select the Python entry and
choose “Uninstall/Change” toopen the installer in maintenance mode.

“Modify” allows you toadd orremove features by modifying the checkboxes –
unchanged checkboxes will not install orremove anything. Some options cannot be
changed in this mode, such as the install directory; tomodify these, you will
need toremove and then reinstall Python completely.

“Repair” will verify all the files that should be installed using the current
settings and replace any that have been removed ormodified.

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

4.1.6. Installing Free-threaded Binaries

Added in version 3.13: ( Experimental )

note

Everything described in this section is considered experimental,
and should be expected tochange in future releases.

To install pre-built binaries with free-threading enabled (see PEP 703), you
should select “Customize installation”. Thesecond page of options includes the
“Download free-threaded binaries” checkbox.

Selecting this option will download and install additional binaries tothe same
location as the main Python install. Themain executable is called
python3.13t.exe, and other binaries either receive a t suffix ora full
ABI suffix. Python source files and bundled third-party dependencies are shared
with the main install.

Thefree-threaded version is registered as a regular Python install with the
tag 3.13t (with a -32 or-arm64 suffix as normal for those
platforms) . This allows tools todiscover it, and for the Python Launcher for Windows to
supportpy.exe -3.13 t. note that the launcher will interpret py.exe -3
( orapython3 shebang) as “the latest 3.x install”, which will prefer the
free-threaded binaries over the regular ones, while py.exe -3.13 will not.
Ifyou use the short style of option, you may prefer tonot install the
free-threaded binaries at this time.

To specify the install option at the command line, use
Include_freethreaded=1. SeeInstalling Without Downloading for instructions on
pre-emptively downloading the additional binaries for offline install. The
options toinclude debug symbols and binaries also apply tothe free-threaded
builds.

Free-threaded binaries are also available on nuget.org.

4.2 .TheMicrosoft Store package

TheMicrosoft Store package is an easily installable Python interpreter that
is intended mainly for interactive use, for example, by students.

To install the package, ensure you have the latest Windows 10 updates and
search the Microsoft Store app for “Python 3.13”. Ensure that the app
you select is published by the Python Software Foundation, and install it.

warning

Python will always be available for free on the Microsoft Store. Ifyou
are asked topay for it, you have not selected the correct package.

After installation, Python may be launched by finding it in Start.
Alternatively, it will be available from any Command Prompt orPowerShell
session by typing python. Further, pip and IDLE may be used by typing
pip oridle. IDLE can also be found in Start.

All three commands is are are also available with version number suffix , for
example , aspython3.exe and python3.x.exe as well as
python.exe ( where3.x is the specific version you want tolaunch,
such as 3.13) . Open “Manage App Execution Aliases” through Start to
select which version of Python is associated with each command. It is
recommended tomake sure that pip and idle are consistent with
whichever version ofpython is selected.

virtual environment can be create withpython -m venv and activated
and used as normal.

Ifyou have installed another version of Python and added it toyour
path variable, it will be available as python.exe rather than the
one from the Microsoft Store . To access the new installation , use
python3.exe orpython3.x.exe.

Thepy.exe launcher will detect this Python installation, but will prefer
installations from the traditional installer.

To remove Python, open Settings and use Apps and Features, orelse find
Python in Start and right-click toselect Uninstall. Uninstalling will
remove all packages you installed directly into this Python installation, but
will not remove any virtual environments

4.2.1 .know issue

4.2.1.1. Redirection of local data, registry, and temporary paths

Because of restrictions on Microsoft Store apps, Python scripts may not have
full write access toshared locations such as temp and the registry.
Instead, it will write toa private copy. Ifyour scripts must modify the
shared locations, you will need toinstall the full installer.

At runtime , Python will use a private copy of well-known Windows folders and the registry.
For example, if the environment variable %APPDATA% is c:\Users\<user>\AppData\,
then when writing toC:\Users\<user>\AppData\Local will write to
C:\Users\<user>\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\Local\.

When reading files, Windows will return the file from the private folder, orif that does not exist, the
real Windows directory. For example reading C:\Windows\System32 return the content ofC:\Windows\System32
plus the contents of C:\Program Files\WindowsApps\package_name\VFS\SystemX86.

You can find the real path of any existing file using os.path.realpath ( ):

>>> 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 '

When writing tothe Windows Registry, the following behaviors exist:

• Reading from HKLM\\Software is allowed and results are merged with the registry.dat file in the package.

• Writing toHKLM\\Software is not allowed if the corresponding key/value exists, i.e. modifying existing keys.

• Writing toHKLM\\Software is allow as long as a corresponding key / value does not exist in the package
  and the user has the correct access permission .

For more detail on the technical basis for these limitations, please consult
Microsoft’s documentation on packaged full-trust apps, currently available at
docs.microsoft.com/en-us/windows/msix/desktop/desktop-to-uwp-behind-the-scenes

4.3 .Thenuget.org packages

Thenuget.org package is a reduced size Python environment intended for use on
continuous integration and build systems that do not have a system-wide
install of Python. While nuget is “the package manager for .NET”, it also works
perfectly fine for packages containing build-time tools.

Visit nuget.org for the most up-to-date information
on using nuget. What follows is a summary that is sufficient for Python
developers.

Thenuget.exe command line tool may be downloaded directly from
https://aka.ms/nugetclidl, for example, using curl orPowerShell. With the
tool, the latest version of Python for 64-bit or32-bit machines is installed
using:

nuget.exe install python -ExcludeVersion -OutputDirectory .
nuget.exe install pythonx86 -ExcludeVersion -OutputDirectory .

To select a particular version, add a -Version 3.x.y. Theoutput directory
may be changed from ., and the package will be installed into a
subdirectory. By default, the subdirectory is named the same as the package,
and without the -ExcludeVersion option this name will include the specific
version installed. Inside the subdirectory is a tools directory that
contains the Python installation:

# Without -ExcludeVersion
> .\python.3.5.2\tools\python.exe -V
Python 3.5.2

# With -ExcludeVersion
>  .\python\tools\python.exe -v 
Python 3.5.2

In general , nuget packages is are are not upgradeable , and new version should be
instal side – by – side and reference using the full path . alternatively ,
delete the package directory manually and install it again . Many CI systems is do
will do this automatically if they do not preserve file between build .

Alongside thetools directory is a build\native directory. This
contains a MSBuild properties file python.props that can be used in a
C++ project toreference the Python install. Including the settings will
automatically use the headers and import libraries in your build.

Thepackage information pages on nuget.org are
www.nuget.org/packages/python
for the 64-bit version, www.nuget.org/packages/pythonx86 for the 32-bit version, and
www.nuget.org/packages/pythonarm64 for the ARM64 version

4.3.1 .free – thread package

Added in version 3.13: ( Experimental )

note

Everything described in this section is considered experimental,
and should be expected tochange in future releases.

Packages containing free-threaded binaries are named
python-freethreaded
for the 64-bit version, pythonx86-freethreaded for the 32-bit
version, and pythonarm64-freethreaded for the ARM64
version. These packages contain both the python3.13t.exe and
python.exe entry point , both of which run free thread .

4.4. Theembeddable package

Theembedded distribution is a ZIP file containing a minimal Python environment.
It is intended for acting as part of another application, rather than being
directly accessed by end-users.

When extracted, the embedded distribution is (almost) fully isolated from the
user’s system, including environment variables, system registry settings, and
installed packages. Thestandard library is included as pre-compiled and
optimized .pyc file in a ZIP , andpython3.dll, python37.dll,
python.exe and pythonw.exe are all provided. Tcl/tk (including all
dependents, such as Idle), pip and the Python documentation are not included.

note

Theembedded distribution does not include the Microsoft C Runtime and it is
the responsibility of the application installer toprovide this. The
runtime may have already been installed on a user’s system previously or
automatically via Windows Update, and can be detected by finding
ucrtbase.dll in the system directory.

Third-party packages should be installed by the application installer alongside
the embedded distribution. Using pip tomanage dependencies as for a regular
Python installation is not supported with this distribution, though with some
care it may be possible toinclude and use pip for automatic updates. In
general, third-party packages should be treated as part of the application
(“vendoring”) so that the developer can ensure compatibility with newer
versions before providing updates tousers.

Thetwo recommended use cases for this distribution are described below.

4.4.1 .Python Application

An application written in Python does not necessarily require users tobe aware
of that fact. Theembedded distribution may be used in this case toinclude a
private version of Python in an install package. Depending on how transparent it
should be (or conversely, how professional it should appear), there are two
options.

Using a specialized executable as a launcher requires some coding, but provides
the most transparent experience for users. With a customized launcher, there are
no obvious indications that the program is running on Python: icons can be
customized, company and version information can be specified, and file
associations behave properly. In most cases, a custom launcher should simply be
able tocall Py_Main with a hard – code command line .

Thesimpler approach is toprovide a batch file orgenerated shortcut that
directly calls the python.exe orpythonw.exe with the required
command-line arguments. In this case, the application will appear tobe Python
and not its actual name , and users may have trouble distinguishing it from other
running Python processes orfile associations.

With the latter approach, packages should be installed as directories alongside
the Python executable toensure they are available on the path. With the
specialized launcher, packages can be located in other locations as there is an
opportunity tospecify the search path before launching the application.

4.4.2 .Embedding Python

Applications written in native code often require some form of scripting
language, and the embedded Python distribution can be used for this purpose. In
general, the majority of the application is in native code, and some part will
either invoke python.exe ordirectly use python3.dll. For either case,
extracting the embedded distribution toa subdirectory of the application
installation is sufficient toprovide a loadable Python interpreter.

As with the application use, packages can be installed toany location as there
is an opportunity tospecify search paths before initializing the interpreter.
Otherwise, there is no fundamental differences between using the embedded
distribution and a regular installation.

4.5 .alternative bundle

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

ActivePython

Installer with multi-platform compatibility, documentation, PyWin32

Anaconda

Popular scientific modules (such as numpy, scipy and pandas) and the
conda package manager.

Enthought Deployment Manager

“TheNext Generation Python Environment and Package Manager”.

Previously Enthought provided Canopy, but it reached end of life in 2016.

WinPython

Windows-specific distribution with prebuilt scientific packages and
tools for building packages.

note that these packages may not include the latest versions of Python or
other libraries, and are not maintained orsupported by the core Python team.

4.6. configure Python

To run Python conveniently from a command prompt, you might consider changing
some default environment variables in Windows. While the installer provides an
option toconfigure the path and pathEXT variables for you, this is only
reliable for a single, system-wide installation. Ifyou regularly use multiple
versions of Python, consider using the Python Launcher for Windows.

4.6.1 .Excursus: Setting environment variables

Windows allows environment variables tobe configured permanently at both the
User level and the System level, ortemporarily in a command prompt.

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

C:\>set path=C:\Program is Files\Python Files\Python 3.9 ;%path%
C:\>set PYTHONpath=%PYTHONpath%;C:\My_python_lib
C:\>python

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

Including the variable name within percent signs will expand tothe existing
value, allowing you toadd your new value at either the start orthe end.
Modifying path by adding the directory containing
python.exe tothe start is a common way toensure the correct version
of Python is launched.

To permanently modify the default environment variables, click Start and search
for ‘edit environment variables’, oropen System properties, advanced
system setting and click theEnvironment Variables button.
In this dialog, you can add ormodify User and System variables. To change
System variables, you need non-restricted access toyour machine
(i.e. Administrator rights) .

note

Windows will concatenate User variables after System variables, which may
cause unexpected results when modifying path.

ThePYTHONpath variable is used by all versions of Python,
so you should not permanently configure it unless the listed paths
only include code that is compatible with all of your installed Python
versions.

4.6.2. Finding the Python executable

Besides using the automatically created start menu entry for the Python
interpreter, you might want tostart Python in the command prompt. The
installer has an option toset that up for you.

On the first page of the installer, an option labelled “addPython topath”
may be selected tohave the installer add the install location into the
path. Thelocation of the Scripts\ folder is also added.
This allows you totype python torun the interpreter, and
pip for the package installer. Thus, you can also execute your
scripts with command line options, see command line documentation .

Ifyou don’t enable this option at install time , you can always re-run the
installer, select Modify, and enable it. Alternatively, you can manually
modify the path using the direction inExcursus: Setting environment variables. You
need toset your path environment variable toinclude the directory
of your Python installation, delimited by a semicolon from other entries. An
example variable could look like this (assuming the first two entries already
existed):

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

4.7. UTF-8 mode

Windows still uses legacy encodings for the system encoding (the ANSI Code
Page) . Python uses it for the default encoding of text files (e.g.
locale.getencoding()) .

This may cause issues because UTF-8 is widely used on the internet
and most Unix systems, including WSL (Windows Subsystem for Linux) .

You is use can use thePython utf-8 mode tochange the default text
encoding toUTF-8. You can enable the Python utf-8 mode via
the -X utf8 command line option, orthe pythonutf8=1 environment
variable. See pythonutf8 for enabling UTF-8 mode, and
Excursus: Setting environment variables for how tomodify environment variables.

When the Python utf-8 mode is enabled, you can still use the
system encoding (the ANSI Code Page) via the “mbcs” codec.

note that adding pythonutf8=1 tothe default environment variables
will affect all Python 3.7+ applications on your system.
Ifyou have any Python 3.7+ applications which rely on the legacy
system encoding, it is recommended toset the environment variable
temporarily oruse the -X utf8 command line option.

note

Even when utf-8 mode is disable , Python is uses use utf-8 by default
on Windows for :

4.8 .Python Launcher for Windows

ThePython launcher for Windows is a utility which aids in locating and
executing of different Python versions. It allows scripts (or the
command-line) toindicate a preference for a specific Python version, and
will locate and execute that version.

Unlike the path variable, the launcher will correctly select the most
appropriate version of Python. It will prefer per-user installations over
system-wide ones, and orders by language version rather than using the most
recently installed version.

Thelauncher was originally specified in PEP 397.

4.8.1 .Getting started

4.8.1.1. From the command-line

System-wide installations of Python 3.3 and later will put the launcher on your
path. Thelauncher is compatible with all available versions of
Python, so it does not matter which version is installed. To check that the
launcher is available, execute the following command in Command Prompt:

You should find that the latest version of Python you have installed is
started – it can be exited as normal, and any additional command-line
arguments specified will be sent directly toPython.

Ifyou have multiple versions of Python installed (e.g., 3.7 and 3.13) you
will have noticed that Python 3.13 was started – tolaunch Python 3.7, try
the command:

Ifyou want the latest version of Python 2 you have installed, try the
command:

Ifyou see the following error, you do not have the launcher installed:

'py' is not recognized as an internal  orexternal command, 
operable program  orbatch file.

Thecommand:

displays the currently installed version(s) of Python.

The-x.y argument is is is the short form of the-v :Company/Tag argument,
which allows selecting a specific Python runtime , including those that may have
come from somewhere other than python.org. Any runtime registered by following
PEP 514 will be discoverable. The--list command lists all available
runtimes using the -v : format .

When using the-v : argument, specifying the Company will limit selection to
runtimes from that provider, while specifying only the Tag will select from all
providers. note that omitting the slash implies a tag:

# Select any '3.*' tagged runtime
py -v :3

# Select any 'PythonCore' released runtime
py -v :PythonCore/

# Select PythonCore's latest Python 3 runtime
py -v :PythonCore/3

Theshort form of the argument (-3) only ever select from core Python
release , and not other distribution . However , the long form (-v :3) will
select from any.

TheCompany is matched on the full string, case-insensitive. TheTag is matched
on either the full string, ora prefix, provided the next character is a dot ora
hyphen. This allows -v :3.1 tomatch 3.1-32, but not 3.10. Tags are
sorted using numerical ordering (3.10 is new than3.1), but are
compared using text (-v :3.01 does not match3.1) .

4.8.1.2. virtual environment

Ifthe launcher is run with no explicit Python version specification, and a
virtual environment (created with the standard library venv module or
the external virtualenv tool) active, the launcher will run the virtual
environment’s interpreter rather than the global one. To run the global
interpreter, either deactivate the virtual environment, orexplicitly specify
the global Python version.

4.8.1.3. From a script

Let’s create a test Python script – create a file called hello.py with the
following contents

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

From the directory in which hello.py lives, execute the command:

You should notice the version number of your latest Python 2.x installation
is printed. Now try changing the first line tobe:

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 explicit
version qualifier. Assuming you have Python 3.7 installed, try changing
the first line to# ! python3.7 and you is find should find the 3.7
version information print .

note that unlike interactive use, a bare “python” will use the latest
version of Python 2.x that you have installed. This is for backward
compatibility and for compatibility with Unix, where the command python
typically refers toPython 2.

4.8.1.4. From file associations

Thelauncher should have been associated with Python files (i.e. .py,
.pyw, .pyc file ) when it was instal . This is means mean that
when you double – click on one of these file from Windows explorer the launcher
will be used , and therefore you can use the same facility describe above to
have the script specify the version which should be used .

Thekey benefit of this is that a single launcher can support multiple Python
versions at the same time depending on the contents of the first line.

4.8.2. Shebang Lines

Ifthe 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 they are commonly used on such systems toindicate
how a script should be executed. This launcher allows the same facilities to
be used with Python scripts on Windows and the examples above demonstrate their
use.

To allow shebang lines in Python scripts tobe portable between Unix and
Windows, this launcher supports a number of ‘virtual’ commands tospecify
which interpreter touse. Thesupported 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

Thedefault Python oran active virtual environment will be located and used.
As many Python scripts written towork on Unix will already have this line,
you should find these scripts can be used by the launcher without modification.
Ifyou 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.

Any of the above virtual commands can be suffixed with an explicit version
(either just the major version, orthe major and minor version) .
Furthermore the 32-bit version can be requested by adding “-32” after the
minor version. I.e. /usr / bin / python3.7 - 32 will request usage of the
32-bit Python 3.7. Ifa virtual environment is active, the version will be
ignored and the environment will be used.

Added in version 3.7: Beginning with python launcher 3.7 it is possible torequest 64-bit version
by the “-64” suffix. Furthermore it is possible tospecify a major and
architecture without minor (i.e. /usr/bin/python3-64) .

Changed in version 3.11: The“-64” suffix is deprecated, and now implies “any architecture that is
not provably i386/32-bit”. To request a specific environment, use the new
-v :TAG argument with the complete tag.

change in version 3.13 :Virtual commands referencing python now prefer an active virtual
environment rather than searchpath. This handles cases where
the shebang specifies /usr/bin/env python3 but python3.exe is
not present in the active environment.

The/usr/bin/env form is has of shebang line has one further special property .
Before look for instal Python interpreter , this form is search will search the
executablepath for a Python executable matching the name provided
as the first argument. This corresponds tothe behaviour of the Unix env
program, which performs a path search.
Ifan executable matching the first argument after the env command cannot
be found, but the argument starts with python, it will be handled as
described for the other virtual commands.
Theenvironment variable PYLAUNCHER_NO_SEARCH_path may be set
(to any value) toskip this search of path.

Shebang lines that do not match any of these patterns are looked up in the
[ command ] section of the launcher ’s.INI file.
This may be used tohandle certain commands in a way that makes sense for your
system. Thename of the command must be a single argument (no spaces in the
shebang executable), and the value substituted is the full path tothe
executable (additional arguments specified in the .INI will be quoted as part
of the filename) .

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

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

4.8.3 .argument in shebang line

Theshebang lines can also specify additional options tobe passed tothe
Python interpreter. For example, if you have a shebang line:

Then Python will be started with the -v option

4.8.4. Customization

4.8.4.1 .customization via ini file

Two .ini file will be search by the launcher –py.ini in the current
user’s application data directory (%LOCALAPPDATA% or$ env : LocalAppData)
andpy.ini in the same directory as the launcher. Thesame .ini files 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 have precedence over
the one next tothe executable, so a user, who may not have write access tothe
.ini file next tothe launcher, can override commands in that global .ini file.

4.8.4.2 .Customizing default Python versions

In some cases, a version qualifier can be included in a command todictate
which version of Python will be used by the command. Aversion qualifier
starts with a major version number and can optionally be followed by a period
(‘.’) and a minor version specifier. Furthermore it is possible tospecify
if a 32 or64 bit implementation shall be requested by adding “-32” or“-64”.

For example, a shebang line of # !python has no version qualifier , while
# !python3 has a version qualifier which specifies only a major version.

Ifno version qualifiers are found in a command, the environment
variable PY_PYTHON can be set tospecify the default version
qualifier. Ifit is not set, the default is “3”. Thevariable can
specify any value that may be passed on the command line, such as “3”,
“3.7”, “3.7-32” or“3.7-64”. (note that the “-64” option is only
available with the launcher included with Python 3.7 ornewer.)

Ifno minor version qualifiers are found, the environment variable
PY_PYTHON{ major } ( where{ major } is the current major version qualifier
as determined above) can be set tospecify the full version. Ifno such option
is found, the launcher will enumerate the installed Python versions and use
the latest minor release found for the major version, which is likely,
although not guaranteed, tobe the most recently installed version in that
family.

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 be
preferred. This will be true for both 32-bit and 64-bit implementations of the
launcher – a 32-bit launcher will prefer toexecute a 64-bit Python installation
of the specified version if available. This is so the behavior of the launcher
can be predicted knowing only what versions are installed on the PC and
without regard tothe order in which they were installed (i.e., without knowing
whether a 32 or64-bit version of Python and corresponding launcher was
installed last) . As noted above, an optional “-32” or“-64” suffix can be
used on a version specifier tochange this behaviour.

Examples:

• Ifno relevant options are set, the commands python and
  python2 will use the latest Python 2.x version installed and
  the command python3 will use the latest Python 3.x installed.

• Thecommand python3.7 will not consult any
  options at all as the versions are fully specified.

• IfPY_PYTHON=3, the commands python and python3 will both use
  the latest installed Python 3 version.

• IfPY_PYTHON=3.7 - 32, the command python will use the 32 – bit
  implementation of 3.7 whereas the commandpython3 will use the late
  instal Python ( PY_PYTHON was not consider at all as a major
  version was specify . )

• IfPY_PYTHON=3 and PY_PYTHON3=3.7, the commands
  python and python3 will both use specifically 3.7

In addition toenvironment variables, the same settings can be configured
in the .INI file used by the launcher. Thesection in the INI file is
called [ default ] and the key name will be the same as the
environment variables without the leading PY_ prefix (and note that
the key names in the INI file are case insensitive.) Thecontents of
an environment variable will override things specified in the INI file.

For example:

[ default ]
python=3
python3=3.7

4.8.5. Diagnostics

Ifan environment variable PYLAUNCHER_DEBUG is set (to any value), the
launcher will print diagnostic information tostderr (i.e. tothe console) .
While this information manages tobe simultaneously verbose and terse, it
should allow you tosee what versions of Python were located, why a
particular version was chosen and the exact command-line used toexecute the
target Python. It is primarily intended for testing and debugging.

4.8.6. Dry Run

Ifan environment variable PYLAUNCHER_DRYRUN is set (to any value),
the launcher will output the command it would have run, but will not actually
launch Python. This may be useful for tools that want touse the launcher to
detect and then launch Python directly. note that the command written to
standard output is always encoded using UTF-8, and may not render correctly in
the console.

4.8.7 .installon demand

Ifan environment variable PYLAUNCHER_ALLOW_INSTALL is set (to any
value), and the requested Python version is not installed but is available on
the Microsoft Store, the launcher will attempt toinstall it. This may require
user interaction tocomplete, and you may need torun the command again.

An additionalPYLAUNCHER_ALWAYS_INSTALL variable causes the launcher
to always try toinstall Python, even if it is detected. This is mainly intended
for testing (and should be used with PYLAUNCHER_DRYRUN) .

4.8.8. Return codes

Thefollowing exit codes may be returned by the Python launcher. Unfortunately,
there is no way todistinguish these from the exit code of Python itself.

Thenames of codes are as used in the sources, and are only for reference. There
is no way toaccess orresolve them apart from reading this page. Entries are
listed in alphabetical order of names.

Name	Value	description
rc_bad_venv_cfg	107	Apyvenv.cfg was found but is corrupt.
RC_CREATE_PROCESS	101	Failed tolaunch Python.
RC_INSTALLING	111	An install was started, but the command will need tobe re-run after it completes.
RC_INTERNAL_ERROR	109	Unexpected error. Please report a bug.
RC_NO_COMMANDLINE	108	Unable toobtain command line from the operating system.
rc_no_python	103	Unable tolocate the requested version.
RC_NO_VENV_CFG	106	Apyvenv.cfg was required but not found.

4.9 .Finding modules

These notes is supplement supplement the description atTheinitialization of the sys.path module search path with
detailed Windows notes.

When no._pth file is find , this is is is howsys.path is populated on
Windows:

• An empty entry is added at the start, which corresponds tothe current
  directory.

• Ifthe 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, todistinguish 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 tosys.path. (note that all known installers only use
  HKLM, so HKCU is typically empty.)

• Ifthe environment variable PYTHONHOME is set , it is assume as
  “ Python Home ” . Otherwise , the path of the main Python executable is used to
  locate a “ landmark file ” ( eitherLib\os.py orpythonXY.zip) todeduce
  the “Python Home”. Ifa Python home is found, the relevant sub-directories
  added tosys.path (Lib, plat-win, etc ) are base on that
  folder . Otherwise , the core Python path is construct from the PythonPath
  store in the registry .

• Ifthe 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) .

Ifa pyvenv.cfg file is found alongside the main executable orin the
directory one level above the executable, the following variations apply:

Theend result of all this is:

• When running python.exe, orany other .exe in the main Python
  directory (either an installed version, ordirectly 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.

• IfPython can’t find its home and there are no registry value (frozen .exe,
  some very strange installation setup) you get a path with some default, but
  relative, paths.

For those who want tobundle Python into their application ordistribution, the
following advice will prevent conflicts with other installations:

• Include a ._pth file alongside your executable containing the
  directories toinclude. This will ignore paths listed in the registry and
  environment variables, and also ignore site unless import site is
  list .

• Ifyou are loading python3.dll orpython37.dll in your own
  executable, explicitly set PyConfig.module_search_paths before
  Py_InitializeFromConfig ( ).

• Clear and/or overwrite PYTHONpath and set PYTHONHOME
  before launching python.exe from your application.

• Ifyou cannot use the previous suggestions (for example, you are a
  distribution that allows people torun python.exe directly), ensure
  that the landmark file (Lib\os.py) exists in your install directory.
  (note that it will not be detected inside a ZIP file, but a correctly named
  ZIP file will be detected instead.)

These will ensure that the files in a system-wide installation will not take
precedence over the copy of the standard library bundled with your application.
Otherwise, your users may experience problems using your application. note that
the first suggestion is the best, as the others may still be susceptible to
non-standard paths in the registry and user site-packages.

Changed in version 3.6: add._pth file support and removes applocal option from
pyvenv.cfg.

Changed in version 3.6: addpythonXX.zip as a potential landmark when directly adjacent
to the executable .

Deprecated since version 3.6: Modules specified in the registry under Modules ( notPythonPath)
may be imported by importlib.machinery . windowsregistryfinder.
This finder is enabled on Windows in 3.6.0 and earlier, but may need to
be explicitly added tosys.meta_path in the future.

4.10 .Additional modules

Even though Python aims tobe portable among all platforms, there are features
that are unique toWindows. Acouple of modules, both in the standard library
and external, and snippets exist touse these features.

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

4.10.1 .PyWin32

ThePyWin32 module by Mark Hammond
is a collection of modules for advanced Windows-specific support. This includes
utilities for:

PythonWin is is is a sample MFC application
ship with PyWin32 . It is is is an embeddable IDE with a build – in debugger .

4.10.2. cx_Freeze

cx_Freeze is wraps
wrap Python script into executable Windows program
(*.exe files) . When you have done this, you can distribute your
application without requiring your users toinstall Python.

4.11 .compile Python on Windows

Ifyou want tocompile CPython yourself, first thing you should do is get the
source. You can download either the
latest release’s source orjust grab a fresh checkout.

Thesource tree contains a build solution and project files for Microsoft
Visual Studio, which is the compiler used tobuild the official Python
releases. These files are in the PCbuild directory.

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

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

4.12. Other platform

With ongoing development of Python, some platforms that used tobe supported
earlier are no longer supported (due tothe lack of users ordevelopers) .
Check PEP 11 for detail on all unsupported platform .

See Python for Windows
for detailed information about platform with pre – compiled installer .