4. 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.

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 (MSI packages) with every
release for many years. These installers are primarily intended to add
a per-user installation of Python, with the core interpreter and
library being used by a single user. The installer is also able to
install for all users of a single machine, and a separate ZIP file is
available for application-local distributions.

As specified in **PEP 11**, a Python release only supports a Windows
platform while Microsoft considers the platform under extended
support. This means that Python 3.12 supports Windows 8.1 and newer.
If you 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.

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

The Microsoft Store package is a simple installation of Python that is
suitable for running scripts and packages, and using IDLE or other
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.

The nuget.org packages are lightweight installations intended for
continuous integration systems. It can be used to build Python
packages or run scripts, but is not updateable and has no user
interface tools.

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


4.1. The full installer
=======================


4.1.1. Installation steps
-------------------------

Four Python 3.12 installers are available for download - two each for
the 32-bit and 64-bit versions of the interpreter. The *web installer*
is a small initial download, and it will automatically download the
required components as necessary. The *offline installer* includes the
components necessary for a default installation and only requires an
internet connection for optional features. See Installing Without
Downloading for other ways to avoid downloading during installation.

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

[image]

If you select "Install Now":

* You will *not* need to be an administrator (unless a system update
  for the C Runtime Library is required or you install the Python
  Launcher for Windows for all users)

* Python will be installed into your user directory

* The Python Launcher for Windows will be installed according to the
  option at the bottom of 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 to install, 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 "Customize
installation". 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 that paths longer than this would not resolve and errors would
result.

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

This allows the "open()" function, the "os" module and most other path
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 the command line, allowing scripted installers to replicate an
installation on many machines without user interaction.  These options
may also be set without suppressing the UI in order to change some of
the defaults.

The following 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 displaying any UI           |
+-----------------------+----------------------------------------------------------+
| /simple               | to prevent user customization                            |
+-----------------------+----------------------------------------------------------+
| /uninstall            | to remove Python (without confirmation)                  |
+-----------------------+----------------------------------------------------------+
| /layout [directory]   | to pre-download all components                           |
+-----------------------+----------------------------------------------------------+
| /log [filename]       | to specify log files location                            |
+-----------------------+----------------------------------------------------------+

All other options are passed as "name=value", where the value is
usually "0" to disable a feature, "1" to enable a feature, or a path.
The full list of available options is shown below.

+-----------------------------+----------------------------------------+----------------------------+
| Name                        | Description                            | Default                    |
|=============================|========================================|============================|
| InstallAllUsers             | Perform a system-wide installation.    | 0                          |
+-----------------------------+----------------------------------------+----------------------------+
| TargetDir                   | The installation directory             | Selected based on          |
|                             |                                        | InstallAllUsers            |
+-----------------------------+----------------------------------------+----------------------------+
| DefaultAllUsersTargetDir    | The default installation directory for | "%ProgramFiles%\Python     |
|                             | all-user installs                      | X.Y" or "%ProgramFiles(x8  |
|                             |                                        | 6)%\Python X.Y"            |
+-----------------------------+----------------------------------------+----------------------------+
| DefaultJustForMeTargetDir   | The default install directory for      | "%LocalAppData%\Programs\  |
|                             | just-for-me installs                   | Python\PythonXY" or "%Loc  |
|                             |                                        | alAppData%\Programs\Pytho  |
|                             |                                        | n\PythonXY-32" or "%Local  |
|                             |                                        | AppData%\Programs\Python\  |
|                             |                                        | PythonXY-64"               |
+-----------------------------+----------------------------------------+----------------------------+
| DefaultCustomTargetDir      | The default custom install directory   | (empty)                    |
|                             | displayed in the UI                    |                            |
+-----------------------------+----------------------------------------+----------------------------+
| AssociateFiles              | Create file associations if the        | 1                          |
|                             | launcher is also installed.            |                            |
+-----------------------------+----------------------------------------+----------------------------+
| CompileAll                  | Compile all ".py" files to ".pyc".     | 0                          |
+-----------------------------+----------------------------------------+----------------------------+
| PrependPath                 | Prepend install and Scripts            | 0                          |
|                             | directories  to "PATH" and add ".PY"   |                            |
|                             | to "PATHEXT"                           |                            |
+-----------------------------+----------------------------------------+----------------------------+
| AppendPath                  | Append install and Scripts directories | 0                          |
|                             | to "PATH" and add ".PY" to "PATHEXT"   |                            |
+-----------------------------+----------------------------------------+----------------------------+
| Shortcuts                   | Create shortcuts for the interpreter,  | 1                          |
|                             | documentation and IDLE if installed.   |                            |
+-----------------------------+----------------------------------------+----------------------------+
| Include_doc                 | Install Python manual                  | 1                          |
+-----------------------------+----------------------------------------+----------------------------+
| Include_debug               | Install debug binaries                 | 0                          |
+-----------------------------+----------------------------------------+----------------------------+
| Include_dev                 | Install developer headers and          | 1                          |
|                             | libraries. Omitting this may lead to   |                            |
|                             | an unusable installation.              |                            |
+-----------------------------+----------------------------------------+----------------------------+
| Include_exe                 | Install "python.exe" and related       | 1                          |
|                             | files. Omitting this may lead to an    |                            |
|                             | unusable installation.                 |                            |
+-----------------------------+----------------------------------------+----------------------------+
| Include_launcher            | Install Python Launcher for Windows.   | 1                          |
+-----------------------------+----------------------------------------+----------------------------+
| InstallLauncherAllUsers     | Installs the launcher for all users.   | 1                          |
|                             | Also requires "Include_launcher" to be |                            |
|                             | set to 1                               |                            |
+-----------------------------+----------------------------------------+----------------------------+
| Include_lib                 | Install standard library and extension | 1                          |
|                             | modules. Omitting this may lead to an  |                            |
|                             | unusable installation.                 |                            |
+-----------------------------+----------------------------------------+----------------------------+
| Include_pip                 | Install bundled pip and setuptools     | 1                          |
+-----------------------------+----------------------------------------+----------------------------+
| Include_symbols             | Install debugging symbols ("*.pdb")    | 0                          |
+-----------------------------+----------------------------------------+----------------------------+
| Include_tcltk               | Install Tcl/Tk support and IDLE        | 1                          |
+-----------------------------+----------------------------------------+----------------------------+
| Include_test                | Install standard library test suite    | 1                          |
+-----------------------------+----------------------------------------+----------------------------+
| Include_tools               | Install utility scripts                | 1                          |
+-----------------------------+----------------------------------------+----------------------------+
| LauncherOnly                | Only installs the launcher. This will  | 0                          |
|                             | override most other options.           |                            |
+-----------------------------+----------------------------------------+----------------------------+
| SimpleInstall               | Disable most install UI                | 0                          |
+-----------------------------+----------------------------------------+----------------------------+
| SimpleInstallDescription    | A custom message to display when the   | (empty)                    |
|                             | simplified install UI is used.         |                            |
+-----------------------------+----------------------------------------+----------------------------+

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
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 omitting the launcher also omits file associations, and is
only recommended for per-user installs when there is also a system-
wide installation that included the launcher.)

The options 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 to a 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 to create 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 to be performed it is very useful to have a
locally cached copy.

Execute the following command from Command Prompt to download all
possible required files.  Remember to substitute "python-3.9.0.exe"
for the actual name of your installer, and to create 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 to hide the progress display.


4.1.5. Modifying an install
---------------------------

Once Python has been installed, you can add or remove features through
the Programs and Features tool that is part of Windows. Select the
Python entry and choose "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 be changed in this mode, such as the install
directory; to modify these, you will need to remove 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 or modified.

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


4.2. The Microsoft Store package
================================

New in version 3.7.2.

The Microsoft 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 3.12". 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. If
  you are asked to pay 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 or
PowerShell session by typing "python". Further, pip and IDLE may be
used by typing "pip" or "idle". IDLE can also be found in Start.

All three commands are also available with version number suffixes,
for example, as "python3.exe" and "python3.x.exe" as well as
"python.exe" (where "3.x" is the specific version you want to launch,
such as 3.12). Open "Manage App Execution Aliases" through Start to
select which version of Python is associated with each command. It is
recommended to make sure that "pip" and "idle" are consistent with
whichever version of "python" is selected.

Virtual environments can be created with "python -m venv" and
activated and used as normal.

If you have installed another version of Python and added it to your
"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" or "python3.x.exe".

The "py.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, or else
find Python in Start and right-click to select Uninstall. Uninstalling
will remove all packages you installed directly into this Python
installation, but will not remove any virtual environments


4.2.1. Known issues
-------------------


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 to shared locations such as "TEMP" and the
registry. Instead, it will write to a private copy. If your scripts
must modify the shared locations, you will need to install 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 to
"C:\Users\<user>\AppData\Local" will write to "C:\Users\<user>\AppDat
a\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\Lo
calCache\Local\".

When reading files, Windows will return the file from the private
folder, or if that does not exist, the real Windows directory. For
example reading "C:\Windows\System32" returns the contents of
"C:\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 to the 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 to "HKLM\\Software" is not allowed if the corresponding
  key/value exists, i.e. modifying existing keys.

* Writing to "HKLM\\Software" is allowed as long as a corresponding
  key/value does not exist in the package and the user has the correct
  access permissions.

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. The nuget.org packages
===========================

New in version 3.5.2.

The nuget.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.

The "nuget.exe" command line tool may be downloaded directly from
"https://aka.ms/nugetclidl", for example, using curl or PowerShell.
With the tool, the latest version of Python for 64-bit or 32-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". The output
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 are not upgradeable, and newer versions
should be installed side-by-side and referenced using the full path.
Alternatively, delete the package directory manually and install it
again. Many CI systems will do this automatically if they do not
preserve files between builds.

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

The package information pages on nuget.org are
www.nuget.org/packages/python for the 64-bit version and
www.nuget.org/packages/pythonx86 for the 32-bit version.


4.4. The embeddable package
===========================

New in version 3.5.

The embedded 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. The standard library is
included as pre-compiled and optimized ".pyc" files in a ZIP, and
"python3.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:

  The embedded distribution does not include the Microsoft C Runtime
  and it is the responsibility of the application installer to provide
  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 to manage dependencies
as for a regular Python installation is not supported with this
distribution, though with some care it may be possible to include 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 to users.

The two recommended use cases for this distribution are described
below.


4.4.1. Python Application
-------------------------

An application written in Python does not necessarily require users to
be aware of that fact. The embedded distribution may be used in this
case to include 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 to call "Py_Main" with
a hard-coded command line.

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

With the latter approach, packages should be installed as directories
alongside the Python executable to ensure they are available on the
path. With the specialized launcher, packages can be located in other
locations as there is an opportunity to specify 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" or directly
use "python3.dll". For either case, extracting the embedded
distribution to a subdirectory of the application installation is
sufficient to provide a loadable Python interpreter.

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


4.5. 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

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

Enthought Deployment Manager
   "The Next 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 or supported by the core
Python team.


4.6. Configuring 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 to configure the PATH and PATHEXT
variables for you, this is only reliable for a single, system-wide
installation.  If you regularly use multiple versions of Python,
consider using the Python Launcher for Windows.


4.6.1. Excursus: Setting environment variables
----------------------------------------------

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

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

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

These changes will apply to any 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 to the
existing value, allowing you to add your new value at either the start
or the end. Modifying "PATH" by adding the directory containing
**python.exe** to the start is a common way to ensure the correct
version of Python is launched.

To permanently modify the default environment variables, click Start
and search for 'edit environment variables', or open System
properties, Advanced system settings and click the Environment
Variables button. 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).

Note:

  Windows will concatenate User variables *after* System variables,
  which may cause unexpected results when modifying "PATH".The
  "PYTHONPATH" 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.

See also:

  https://docs.microsoft.com/en-us/windows/win32/procthread
  /environment-variables
     Overview of environment variables on Windows

  https://docs.microsoft.com/en-us/windows-server/administration
  /windows-commands/set_1
     The "set" command, for temporarily modifying environment
     variables

  https://docs.microsoft.com/en-us/windows-server/administration
  /windows-commands/setx
     The "setx" command, for permanently modifying environment
     variables


4.6.2. Finding the Python executable
------------------------------------

Changed in version 3.5.

Besides using the automatically created start menu entry for the
Python interpreter, you might want to start Python in the command
prompt. The installer 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 the "PATH".  The location of the "Scripts\" folder is also added.
This allows you to type **python** to run the interpreter, and **pip**
for the package installer. Thus, you can also execute your scripts
with command line options, see Command line documentation.

If you 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 directions in Excursus: Setting
environment variables.  You need to set your "PATH" environment
variable to include 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
===============

New in version 3.7.

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 can use the Python UTF-8 Mode to change the default text encoding
to UTF-8. You can enable the Python UTF-8 Mode via the "-X utf8"
command line option, or the "PYTHONUTF8=1" environment variable.  See
"PYTHONUTF8" for enabling UTF-8 mode, and Excursus: Setting
environment variables for how to modify 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" to the default environment variables
will affect all Python 3.7+ applications on your system. If you have
any Python 3.7+ applications which rely on the legacy system encoding,
it is recommended to set the environment variable temporarily or use
the "-X utf8" command line option.

Note:

  Even when UTF-8 mode is disabled, Python uses UTF-8 by default on
  Windows for:

  * Console I/O including standard I/O (see **PEP 528** for details).

  * The *filesystem encoding* (see **PEP 529** for details).


4.8. Python Launcher for Windows
================================

New in version 3.3.

The Python launcher for Windows is a utility which aids in locating
and executing of different Python versions.  It allows scripts (or the
command-line) to indicate 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.

The launcher was originally specified in **PEP 397**.


4.8.1. Getting started
----------------------


4.8.1.1. From the command-line
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Changed in version 3.6.

System-wide installations of Python 3.3 and later will put the
launcher on your "PATH". The launcher 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:

   py

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 to Python.

If you have multiple versions of Python installed (e.g., 3.7 and 3.12)
you will have noticed that Python 3.12 was started - to launch Python
3.7, try the command:

   py -3.7

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

   py -2

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

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

The command:

   py --list

displays the currently installed version(s) of Python.

The "-x.y" argument 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

The short form of the argument ("-3") only ever selects from core
Python releases, and not other distributions. However, the longer form
("-V:3") will select from any.

The Company is matched on the full string, case-insenitive. The Tag is
matched oneither the full string, or a prefix, provided the next
character is a dot or a hyphen. This allows "-V:3.1" to match
"3.1-32", but not "3.10". Tags are sorted using numerical ordering
("3.10" is newer than "3.1"), but are compared using text ("-V:3.01"
does not match "3.1").


4.8.1.2. Virtual environments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

New in version 3.5.

If the 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, or explicitly 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:

   py hello.py

You should notice the version number of your latest Python 2.x
installation is 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 explicit version qualifier.  Assuming you have Python 3.7
installed, try changing the first line to "#! python3.7" and you
should find the 3.7 version information printed.

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 to Python 2.


4.8.1.4. From file associations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The launcher should have been associated with Python files (i.e.
".py", ".pyw", ".pyc" files) when it was installed.  This means that
when you double-click on one of these files from Windows explorer the
launcher will be used, and therefore you can use the same facilities
described above to have the script specify the version which should be
used.

The key 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
--------------------

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
native support for such lines and they are commonly used on such
systems to indicate 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 to be portable between Unix
and Windows, this launcher supports a number of 'virtual' commands to
specify which 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
written to work on Unix will already have this line, you should find
these scripts can be used by the launcher without modification.  If
you 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, or the 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.

New in version 3.7: Beginning with python launcher 3.7 it is possible
to request 64-bit version by the "-64" suffix. Furthermore it is
possible to specify 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.

The "/usr/bin/env" form of shebang line has one further special
property. Before looking for installed Python interpreters, this form
will search the executable "PATH" for a Python executable matching the
name provided as the first argument. This corresponds to the behaviour
of the Unix "env" program, which performs a "PATH" search. If an
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. 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 your system.
The name of the command must be a single argument (no spaces in the
shebang executable), and the value substituted is the full path to the
executable (additional arguments specified in the .INI will be quoted
as part of the filename).

   [commands]
   /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 or relative to the 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 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
the Python 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
--------------------


4.8.4.1. Customization via INI files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Two .ini files will be searched by the launcher - "py.ini" in the
current user's application data directory ("%LOCALAPPDATA%" or
"$env:LocalAppData") and "py.ini" in the same directory as the
launcher. The same .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 to the executable, so a user, who may not
have write access to the .ini file next to the 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 to
dictate which version of Python will be used by the command. A version
qualifier starts with a major version number and can optionally be
followed by a period ('.') and a minor version specifier. Furthermore
it is possible to specify if a 32 or 64 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.

If no version qualifiers are found in a command, the environment
variable "PY_PYTHON" can be set to specify the default version
qualifier. If it is not set, the default is "3". The variable 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 or newer.)

If no 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 to specify the full version.
If no 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, to be 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 to
execute 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
to the order in which they were installed (i.e., without knowing
whether a 32 or 64-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 to change this behaviour.

Examples:

* If no 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.

* The command "python3.7" will not consult any options at all as the
  versions are fully specified.

* If "PY_PYTHON=3", the commands "python" and "python3" will both use
  the latest installed Python 3 version.

* If "PY_PYTHON=3.7-32", the command "python" will use the 32-bit
  implementation of 3.7 whereas the command "python3" will use the
  latest installed Python (PY_PYTHON was not considered at all as a
  major version was specified.)

* If "PY_PYTHON=3" and "PY_PYTHON3=3.7", the commands "python" and
  "python3" will both use specifically 3.7

In addition to environment variables, the same settings can be
configured in the .INI file used by the launcher.  The section in the
INI file is called "[defaults]" 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.)  The
contents of an environment variable will override things specified in
the INI file.

For example:

* Setting "PY_PYTHON=3.7" is equivalent to the INI file containing:

   [defaults]
   python=3.7

* Setting "PY_PYTHON=3" and "PY_PYTHON3=3.7" is equivalent to the INI
  file containing:

   [defaults]
   python=3
   python3=3.7


4.8.5. Diagnostics
------------------

If an environment variable "PYLAUNCHER_DEBUG" is set (to any value),
the launcher will print diagnostic information to stderr (i.e. to the
console). While this information manages to be simultaneously verbose
*and* terse, it should allow you to see what versions of Python were
located, why a particular version was chosen and the exact command-
line used to execute the target 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
actually launch Python. This may be useful for tools that want to use
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. Install on demand
------------------------

If an 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 to install
it. This may require user interaction to complete, and you may need to
run the command again.

An additional "PYLAUNCHER_ALWAYS_INSTALL" variable causes the launcher
to always try to install Python, even if it is detected. This is
mainly intended for 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. There is no way to access or resolve them apart from
reading this page. Entries are listed in alphabetical order of names.

+---------------------+---------+-------------------------------------------------+
| Name                | Value   | Description                                     |
|=====================|=========|=================================================|
| RC_BAD_VENV_CFG     | 107     | A "pyvenv.cfg" was found but is corrupt.        |
+---------------------+---------+-------------------------------------------------+
| RC_CREATE_PROCESS   | 101     | Failed to launch Python.                        |
+---------------------+---------+-------------------------------------------------+
| RC_INSTALLING       | 111     | An install was started, but the command will    |
|                     |         | need to be re-run after it completes.           |
+---------------------+---------+-------------------------------------------------+
| RC_INTERNAL_ERROR   | 109     | Unexpected error. Please report a bug.          |
+---------------------+---------+-------------------------------------------------+
| RC_NO_COMMANDLINE   | 108     | Unable to obtain command line from the          |
|                     |         | operating system.                               |
+---------------------+---------+-------------------------------------------------+
| RC_NO_PYTHON        | 103     | Unable to locate the requested version.         |
+---------------------+---------+-------------------------------------------------+
| RC_NO_VENV_CFG      | 106     | A "pyvenv.cfg" was required but not found.      |
+---------------------+---------+-------------------------------------------------+


4.9. Finding modules
====================

These notes supplement the description at The initialization of the
sys.path module search path with detailed Windows notes.

When no "._pth" file is found, 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" (either "Lib\os.py" or
  "pythonXY.zip") 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).

If a "pyvenv.cfg" file is found alongside the main executable or in
the directory one level above the executable, the following variations
apply:

* If "home" is an absolute path and "PYTHONHOME" is not set, this path
  is used instead of the path to the main executable when deducing the
  home location.

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 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 to bundle Python into their application or
distribution, the following advice will prevent conflicts with other
installations:

* Include a "._pth" file alongside your executable containing the
  directories to include. This will ignore paths listed in the
  registry and environment variables, and also ignore "site" unless
  "import site" is listed.

* If you are loading "python3.dll" or "python37.dll" in your own
  executable, explicitly call "Py_SetPath()" or (at least)
  "Py_SetProgramName()" before "Py_Initialize()".

* Clear and/or overwrite "PYTHONPATH" and set "PYTHONHOME" before
  launching "python.exe" from your application.

* If you cannot use the previous suggestions (for example, you are a
  distribution that allows people to run "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:

   * Adds "._pth" file support and removes "applocal" option from
     "pyvenv.cfg".

   * Adds "python*XX*.zip" as a potential landmark when directly
     adjacent to the executable.

Deprecated since version 3.6:

   Modules specified in the registry under "Modules" (not
   "PythonPath") 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 to "sys.meta_path" in the future.


4.10. 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.


4.10.1. 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


4.10.2. cx_Freeze
-----------------

cx_Freeze 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.


4.11. 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.

The source tree contains a build solution and project files for
Microsoft Visual Studio, which is the compiler used to build 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 Platforms
=====================

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.

* Windows CE is no longer supported since Python 3 (if it ever was).

* The Cygwin installer offers to install the Python interpreter as
  well

See Python for Windows for detailed information about platforms with
pre-compiled installers.
