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    .. _using-on-windows:
    
    *************************
     Using Python on Windows
    *************************
    
    .. sectionauthor:: Robert Lehmann <lehmannro@gmail.com>
    
    .. sectionauthor:: Steve Dower <steve.dower@microsoft.com>
    
    
    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
    
    <https://www.python.org/download/releases/>`_ 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 |version| 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.
    
    :ref:`windows-full` contains all components and is the best option for
    developers using Python for any kind of project.
    
    :ref:`windows-store` 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.
    
    :ref:`windows-nuget` 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.
    
    :ref:`windows-embeddable` is a minimal package of Python suitable for
    embedding into a larger application.
    
    
    .. _windows-full:
    
    The full installer
    ==================
    
    Installation steps
    
    ------------------
    
    
    Four Python |version| 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 :ref:`install-layout-option` for other ways to avoid downloading
    during installation.
    
    
    After starting the installer, one of two options may be selected:
    
    
    .. image:: win_installer.png
    
    
    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 :ref:`launcher` for all
      users)
    
    * Python will be installed into your user directory
    
    * The :ref:`launcher` 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 :envvar:`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 :ref:`launcher` 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 :envvar:`PATH`
    * Shortcuts are available for all users
    
    
    .. _max-path:
    
    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 :func:`open` function, the :mod:`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.
    
    .. versionchanged:: 3.6
    
       Support for long paths was enabled in Python.
    
    
    .. _install-quiet-option:
    
    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   | :file:`%ProgramFiles%\\\ |
    |                           | for all-user installs                | Python X.Y` or :file:`\  |
    |                           |                                      | %ProgramFiles(x86)%\\\   |
    |                           |                                      | Python X.Y`              |
    +---------------------------+--------------------------------------+--------------------------+
    | DefaultJustForMeTargetDir | The default install directory for    | :file:`%LocalAppData%\\\ |
    |                           | just-for-me installs                 | Programs\\PythonXY` or   |
    |                           |                                      | :file:`%LocalAppData%\\\ |
    
    |                           |                                      | Programs\\PythonXY-32` or|
    |                           |                                      | :file:`%LocalAppData%\\\ |
    |                           |                                      | Programs\\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         | 0                        |
    
    |                           | ``.pyc``.                            |                          |
    
    +---------------------------+--------------------------------------+--------------------------+
    
    | PrependPath               | Prepend install and Scripts          | 0                        |
    |                           | directories  to :envvar:`PATH` and   |                          |
    |                           | add ``.PY`` to :envvar:`PATHEXT`     |                          |
    +---------------------------+--------------------------------------+--------------------------+
    | AppendPath                | Append install and Scripts           | 0                        |
    |                           | directories  to :envvar:`PATH` and   |                          |
    |                           | add ``.PY`` to :envvar:`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                            |                          |
    +---------------------------+--------------------------------------+--------------------------+
    | Include_exe               | Install :file:`python.exe` and       | 1                        |
    |                           | related files                        |                          |
    +---------------------------+--------------------------------------+--------------------------+
    | Include_launcher          | Install :ref:`launcher`.             | 1                        |
    +---------------------------+--------------------------------------+--------------------------+
    
    | InstallLauncherAllUsers   | Installs :ref:`launcher` for all     | 1                        |
    |                           | users.                               |                          |
    +---------------------------+--------------------------------------+--------------------------+
    
    | Include_lib               | Install standard library and         | 1                        |
    |                           | extension modules                    |                          |
    +---------------------------+--------------------------------------+--------------------------+
    | 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     | 0                        |
    |                           | will 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>
    
    
    .. _install-layout-option:
    
    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.
    
    
    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
    :ref:`launcher`, which has its own entry in Programs and Features.
    
    The Microsoft Store package
    ===========================
    
    .. versionadded:: 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 |version|". 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 |version|). 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
    
    
    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 :envvar:`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 :envvar:`%APPDATA%` is :file:`c:\\Users\\<user>\\AppData\\`,
    then when writing to :file:`C:\\Users\\<user>\\AppData\\Local` will write to
    :file:`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, or if that does not exist, the
    real Windows directory. For example reading :file:`C:\\Windows\\System32` returns the contents of :file:`C:\\Windows\\System32`
    plus the contents of :file:`C:\\Program Files\\WindowsApps\\package_name\\VFS\\SystemX86`.
    
    You can find the real path of any existing file using :func:`os.path.realpath`:
    
    .. code-block:: python
    
      >>> 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 :file:`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
    <https://docs.microsoft.com/en-us/windows/msix/desktop/desktop-to-uwp-behind-the-scenes>`_
    
    
    
    .. _windows-nuget:
    
    The nuget.org packages
    ======================
    
    .. versionadded:: 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 <https://www.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:
    
    .. code-block:: doscon
    
    
       # 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 <https://www.nuget.org/packages/python>`_
    for the 64-bit version and `www.nuget.org/packages/pythonx86
    <https://www.nuget.org/packages/pythonx86>`_ for the 32-bit version.
    
    
    .. _windows-embeddable:
    
    The embeddable package
    ======================
    
    .. versionadded:: 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
    dependants, such as Idle), pip and the Python documentation are not included.
    
    .. note::
    
        The embedded distribution does not include the `Microsoft C Runtime
    
        <https://docs.microsoft.com/en-US/cpp/windows/latest-supported-vc-redist#visual-studio-2015-2017-2019-and-2022>`_ 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.
    
    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.
    
    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.
    
    
    
    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 <https://www.activestate.com/activepython/>`_
    
        Installer with multi-platform compatibility, documentation, PyWin32
    
    
    `Anaconda <https://www.anaconda.com/download/>`_
    
        Popular scientific modules (such as numpy, scipy and pandas) and the
        ``conda`` package manager.
    
    
    `Enthought Deployment Manager <https://www.enthought.com/edm/>`_
        "The Next Generation Python Environment and Package Manager".
    
        Previously Enthought provided Canopy, but it `reached end of life in 2016
        <https://support.enthought.com/hc/en-us/articles/360038600051-Canopy-GUI-end-of-life-transition-to-the-Enthought-Deployment-Manager-EDM-and-Visual-Studio-Code>`_.
    
    `WinPython <https://winpython.github.io/>`_
        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.
    
    
    
    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 :ref:`launcher`.
    
    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
    
    :command:`set` command:
    
    .. code-block:: doscon
    
        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 :envvar:`PATH` by adding the directory containing
    :program:`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, :guilabel:`Advanced
    system settings` and click the :guilabel:`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).
    
    
        Windows will concatenate User variables *after* System variables, which may
        cause unexpected results when modifying :envvar:`PATH`.
    
        The :envvar:`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
    
        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
    
    Finding the Python executable
    -----------------------------
    
    
    .. versionchanged:: 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
    
    :envvar:`PATH`.  The location of the :file:`Scripts\\` folder is also added.
    This allows you to type :command:`python` to run the interpreter, and
    
    :command:`pip` for the package installer. Thus, you can also execute your
    
    scripts with command line options, see :ref:`using-on-cmdline` 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 :envvar:`PATH` using the directions in :ref:`setting-envvars`.  You
    need to set your :envvar:`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
    
    .. _win-utf8-mode:
    
    UTF-8 mode
    ==========
    
    .. versionadded:: 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.
    
    :func:`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 :ref:`Python UTF-8 Mode <utf8-mode>` to change the default text
    encoding to UTF-8. You can enable the :ref:`Python UTF-8 Mode <utf8-mode>` via
    the ``-X utf8`` command line option, or the ``PYTHONUTF8=1`` environment
    variable.  See :envvar:`PYTHONUTF8` for enabling UTF-8 mode, and
    :ref:`setting-envvars` for how to modify environment variables.
    
    When the :ref:`Python UTF-8 Mode <utf8-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 :term:`filesystem encoding <filesystem encoding and error handler>`
         (see :pep:`529` for details).
    
    .. _launcher:
    
    Python Launcher for Windows
    ===========================
    
    .. versionadded:: 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 :envvar:`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`.
    
    
    Getting started
    ---------------
    
    From the command-line
    ^^^^^^^^^^^^^^^^^^^^^
    
    
    System-wide installations of Python 3.3 and later will put the launcher on your
    :envvar:`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::
    
    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 |version|) you
    will have noticed that Python |version| was started - to launch Python 3.7, try
    the command::
    
    If you want the latest version of Python 2 you have installed, try the
    command::
    
    You should find the latest version of Python 3.x starts.
    
    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.
    
    Per-user installations of Python do not add the launcher to :envvar:`PATH`
    unless the option was selected on installation.
    
    
    displays the currently installed version(s) of Python.
    
    .. versionadded:: 3.5
    
    
    If the launcher is run with no explicit Python version specification, and a
    virtual environment (created with the standard library :mod:`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.
    
    
    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 |version|
    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.
    
    
    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.
    
    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 python``
    * ``/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.
    
    
    .. versionadded:: 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``).
    
    .. versionchanged:: 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 :envvar:`PATH` for a Python executable. This corresponds to the
    behaviour of the Unix ``env`` program, which performs a :envvar:`PATH` search.
    
    
    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
    
    Customization
    -------------
    
    Customization via INI files
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    
    
    Two .ini files will be searched by the launcher - ``py.ini`` in the current
    user's "application data" directory (i.e. the directory returned by calling the
    
    Windows function ``SHGetFolderPath`` with ``CSIDL_LOCAL_APPDATA``) 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
    
    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.
    
    
    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 :envvar:`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:
    
    * Setting ``PY_PYTHON=3`` and ``PY_PYTHON3=3.7`` is equivalent to the INI file
    
    
      [defaults]
      python=3
    
    
    Diagnostics
    -----------
    
    
    If an environment variable :envvar:`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.
    
    Dry Run
    -------
    
    If an environment variable :envvar:`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.
    
    Install on demand