PEP: 514
Title: Python registration in the Windows registry
Author: Steve Dower <steve.dower@python.org>
BDFL-Delegate: Paul Moore <p.f.moore@gmail.com>
Status: Active
Type: Informational
Created: 02-Feb-2016
Post-History: 02-Feb-2016, 01-Mar-2016, 18-Jul-2016
Resolution: https://mail.python.org/pipermail/python-dev/2016-July/145697.html

Abstract
========

This PEP defines a schema for the Python registry key to allow third-party
installers to register their installation, and to allow tools and applications
to detect and correctly display all Python environments on a user's machine. No
implementation changes to Python are proposed with this PEP.

Python environments are not required to be registered unless they want to be
automatically discoverable by external tools. As this relates to Windows only,
these tools are expected to be predominantly GUI applications. However, console
applications may also make use of the registered information. This PEP covers
the information that may be made available, but the actual presentation and use
of this information is left to the tool designers.

The schema matches the registry values that have been used by the official
installer since at least Python 2.5, and the resolution behaviour matches the
behaviour of the official Python releases. Some backwards compatibility rules
are provided to ensure tools can correctly detect versions of CPython that do
not register full information.

Motivation
==========

When installed on Windows, the official Python installer creates a registry key
for discovery and detection by other applications. This allows tools such as
installers or IDEs to automatically detect and display a user's Python
installations. For example, the :pep:`397` ``py.exe`` launcher and editors such as
PyCharm and Visual Studio already make use of this information.

Third-party installers, such as those used by distributions, typically create
identical keys for the same purpose. Most tools that use the registry to detect
Python installations only inspect the keys used by the official installer. As a
result, third-party installations that wish to be discoverable will overwrite
these values, often causing users to "lose" their original Python installation.

By describing a layout for registry keys that allows third-party installations
to register themselves uniquely, as well as providing tool developers guidance
for discovering all available Python installations, these collisions should be
prevented. We also take the opportunity to add some well-known metadata so that
more information can be presented to users.

Definitions
===========

A "registry key" is the equivalent of a file-system path into the registry. Each
key may contain "subkeys" (keys nested within keys) and "values" (named and
typed attributes attached to a key). These are used on Windows to store settings
in much the same way that directories containing configuration files would work.

``HKEY_CURRENT_USER`` is the root of settings for the currently logged-in user,
and this user can generally read and write all settings under this root.

``HKEY_LOCAL_MACHINE`` is the root of settings for all users. Generally, any
user can read these settings but only administrators can modify them. It is
typical for values under ``HKEY_CURRENT_USER`` to take precedence over those in
``HKEY_LOCAL_MACHINE``.

On 64-bit Windows, ``HKEY_LOCAL_MACHINE\Software\Wow6432Node`` is a special key
that 32-bit processes transparently read and write to rather than accessing the
``Software`` key directly.

Further documentation regarding registry redirection on Windows is available
from the MSDN Library [1]_.

Structure
=========

We consider there to be a single collection of Python environments on a machine,
where the collection may be different for each user of the machine. There are
three potential registry locations where the collection may be stored based on
the installation options of each environment::

    HKEY_CURRENT_USER\Software\Python\<Company>\<Tag>
    HKEY_LOCAL_MACHINE\Software\Python\<Company>\<Tag>
    HKEY_LOCAL_MACHINE\Software\Wow6432Node\Python\<Company>\<Tag>

Official Python releases use ``PythonCore`` for Company, and the value of
``sys.winver`` for Tag. The Company ``PyLauncher`` is reserved. Other registered
environments may use any values for Company and Tag. Recommendations are made
later in this document.

Company-Tag pairs are case-insensitive, and uniquely identify each environment.
Depending on the purpose and intended use of a tool, there are two suggested
approaches for resolving conflicts between Company-Tag pairs.

Tools that list every installed environment may choose to include those
even where the Company-Tag pairs match. They should ensure users can easily
identify whether the registration was per-user or per-machine, and which
registration has the higher priority.

Tools that aim to select a single installed environment from all registered
environments based on the Company-Tag pair, such as the ``py.exe`` launcher,
should always select the environment registered in ``HKEY_CURRENT_USER``
instead of the matching one in ``HKEY_LOCAL_MACHINE``.

Conflicts between ``HKEY_LOCAL_MACHINE\Software\Python`` and
``HKEY_LOCAL_MACHINE\Software\Wow6432Node\Python`` should only occur when both
64-bit and 32-bit versions of an interpreter have the same Tag. In this case,
the tool should select whichever is more appropriate for its use.

If a tool is able to determine from the provided information (or lack thereof)
that it cannot use a registered environment, there is no obligation to present
it to users.

Except as discussed in the section on backwards compatibility, Company and Tag
values are considered opaque to tools, and no information about the interpreter
should be inferred from the text. However, some tools may display the Company
and Tag values to users, so ideally the Tag will be able to help users identify
the associated environment.

Python environments are not required to register themselves unless they want to
be automatically discoverable by external tools.

Backwards Compatibility
-----------------------

Python 3.4 and earlier did not distinguish between 32-bit and 64-bit builds in
``sys.winver``. As a result, it is not possible to have valid side-by-side
installations of both 32-bit and 64-bit interpreters under this scheme since it
would result in duplicate Tags.

To ensure backwards compatibility, applications should treat environments listed
under the following two registry keys as distinct, even when the Tag matches::

    HKEY_LOCAL_MACHINE\Software\Python\PythonCore\<Tag>
    HKEY_LOCAL_MACHINE\Software\Wow6432Node\Python\PythonCore\<Tag>

Environments listed under ``HKEY_CURRENT_USER`` may be treated as distinct from
both of the above keys, potentially resulting in three environments discovered
using the same Tag. Alternatively, a tool may determine whether the per-user
environment is 64-bit or 32-bit and give it priority over the per-machine
environment, resulting in a maximum of two discovered environments.

It is not possible to detect side-by-side installations of both 64-bit and
32-bit versions of Python prior to 3.5 when they have been installed for the
current user. Python 3.5 and later always uses different Tags for 64-bit and
32-bit versions.

The following section describe user-visible information that may be registered.
For Python 3.5 and earlier, none of this information is available, but
alternative defaults are specified for the ``PythonCore`` key.

Environments registered under other Company names have no backward compatibility
requirements and must use distinct Tags to support side-by-side installations.
Tools consuming these registrations are not required to disambiguate tags other
than by preferring the user's setting.

Company
-------

The Company part of the key is intended to group related environments and to
ensure that Tags are namespaced appropriately. The key name should be
alphanumeric without spaces and likely to be unique. For example, a trademarked
name (preferred), a hostname, or as a last resort, a UUID would be appropriate::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp
    HKEY_CURRENT_USER\Software\Python\www.example.com
    HKEY_CURRENT_USER\Software\Python\6C465E66-5A8C-4942-9E6A-D29159480C60

The company name ``PyLauncher`` is reserved for the :pep:`397` launcher
(``py.exe``). It does not follow this convention and should be ignored by tools.

If a string value named ``DisplayName`` exists, it should be used to identify
the environment manufacturer/developer/distributor to users. Otherwise, the name
of the key should be used. (For ``PythonCore``, the default display name is
"Python Software Foundation".)

If a string value named ``SupportUrl`` exists, it may be displayed or otherwise
used to direct users to a web site related to the environment. (For
``PythonCore``, the default support URL is "http://www.python.org/".)

A complete example may look like::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp
        (Default) = (value not set)
        DisplayName = "Example Corp"
        SupportUrl = "http://www.example.com"

Tag
---

The Tag part of the key is intended to uniquely identify an environment within
those provided by a single company. The key name should be alphanumeric without
spaces and stable across installations. For example, the Python language
version, a UUID or a partial/complete hash would be appropriate, while a Tag
based on the install directory or some aspect of the current machine may not.
For example::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\examplepy
    HKEY_CURRENT_USER\Software\Python\ExampleCorp\3.6
    HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66

It is expected that some tools will require users to type the Tag into a command
line, and that the Company may be optional provided the Tag is unique across all
Python installations. Short, human-readable and easy to type Tags are
recommended, and if possible, select a value likely to be unique across all
other Companies.

If a string value named ``DisplayName`` exists, it should be used to identify
the environment to users. Otherwise, the name of the key should be used. (For
``PythonCore``, the default is "Python " followed by the Tag.)

If a string value named ``SupportUrl`` exists, it may be displayed or otherwise
used to direct users to a web site related to the environment. (For
``PythonCore``, the default is "http://www.python.org/".)

If a string value named ``Version`` exists, it should be used to identify the
version of the environment. This is independent from the version of Python
implemented by the environment. (For ``PythonCore``, the default is the first
three characters of the Tag.)

If a string value named ``SysVersion`` exists, it must be in ``x.y`` or
``x.y.z`` format matching the version returned by ``sys.version_info`` in the
interpreter. If omitted, the Python version is unknown. (For ``PythonCore``,
the default is the first three characters of the Tag.)

If a string value named ``SysArchitecture`` exists, it must match the first
element of the tuple returned by ``platform.architecture()``. Typically, this
will be "32bit" or "64bit". If omitted, the architecture is unknown. (For
``PythonCore``, the architecture is "32bit" when registered under
``HKEY_LOCAL_MACHINE\Software\Wow6432Node\Python`` *or* anywhere on a 32-bit
operating system, "64bit" when registered under
``HKEY_LOCAL_MACHINE\Software\Python`` on a 64-bit machine, and unknown when
registered under ``HKEY_CURRENT_USER``.)

Note that each of these values is recommended, but optional. Omitting
``SysVersion`` or ``SysArchitecture`` may prevent some tools from correctly
supporting the environment. A complete example may look like this::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\examplepy
        (Default) = (value not set)
        DisplayName = "Example Py Distro 3"
        SupportUrl = "http://www.example.com/distro-3"
        Version = "3.0.12345.0"
        SysVersion = "3.6.0"
        SysArchitecture = "64bit"

InstallPath
-----------

Beneath the environment key, an ``InstallPath`` key must be created. This key is
always named ``InstallPath``, and the default value must match ``sys.prefix``::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\3.6\InstallPath
        (Default) = "C:\ExampleCorpPy36"

If a string value named ``ExecutablePath`` exists, it must be the full path to
the ``python.exe`` (or equivalent) executable. If omitted, the environment is
not executable. (For ``PythonCore``, the default is the ``python.exe`` file in
the directory referenced by the ``(Default)`` value.)

If a string value named ``ExecutableArguments`` exists, tools should use the
value as the first arguments when executing ``ExecutablePath``. Tools may add
other arguments following these, and will reasonably expect standard Python
command line options to be available.

If a string value named ``WindowedExecutablePath`` exists, it must be a path to
the ``pythonw.exe`` (or equivalent) executable. If omitted, the default is the
value of ``ExecutablePath``, and if that is omitted the environment is not
executable. (For ``PythonCore``, the default is the ``pythonw.exe`` file in the
directory referenced by the ``(Default)`` value.)

If a string value named ``WindowedExecutableArguments`` exists, tools should use
the value as the first arguments when executing ``WindowedExecutablePath``.
Tools may add other arguments following these, and will reasonably expect
standard Python command line options to be available.

A complete example may look like::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\examplepy\InstallPath
        (Default) = "C:\ExampleDistro30"
        ExecutablePath = "C:\ExampleDistro30\ex_python.exe"
        ExecutableArguments = "--arg1"
        WindowedExecutablePath = "C:\ExampleDistro30\ex_pythonw.exe"
        WindowedExecutableArguments = "--arg1"

Help
----

Beneath the environment key, a ``Help`` key may be created. This key is always
named ``Help`` if present and has no default value.

Each subkey of ``Help`` specifies a documentation file, tool, or URL associated
with the environment. The subkey may have any name, and the default value is a
string appropriate for passing to ``os.startfile`` or equivalent.

If a string value named ``DisplayName`` exists, it should be used to identify
the help file to users. Otherwise, the key name should be used.

A complete example may look like::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66\Help
        Python\
            (Default) = "C:\ExampleDistro30\python36.chm"
            DisplayName = "Python Documentation"
        Extras\
            (Default) = "http://www.example.com/tutorial"
            DisplayName = "Example Distro Online Tutorial"

Other Keys
----------

All other subkeys under a Company-Tag pair are available for private use.

Official CPython releases have traditionally used certain keys in this space to
determine the location of the Python standard library and other installed
modules. This behaviour is retained primarily for backward compatibility.
However, as the code that reads these values is embedded into the interpreter,
third-party distributions may be affected by values written into ``PythonCore``
if using an unmodified interpreter.

Sample Code
===========

This sample code enumerates the registry and displays the available Company-Tag
pairs that could be used to launch an environment and the target executable. It
only shows the most-preferred target for the tag. Backwards-compatible handling
of ``PythonCore`` is omitted but shown in a later example::

    # Display most-preferred environments.
    # Assumes a 64-bit operating system
    # Does not correctly handle PythonCore compatibility

    import winreg

    def enum_keys(key):
        i = 0
        while True:
            try:
                yield winreg.EnumKey(key, i)
            except OSError:
                break
            i += 1

    def get_value(key, value_name):
        try:
            return winreg.QueryValue(key, value_name)
        except FileNotFoundError:
            return None

    seen = set()
    for hive, key, flags in [
        (winreg.HKEY_CURRENT_USER, r'Software\Python', 0),
        (winreg.HKEY_LOCAL_MACHINE, r'Software\Python', winreg.KEY_WOW64_64KEY),
        (winreg.HKEY_LOCAL_MACHINE, r'Software\Python', winreg.KEY_WOW64_32KEY),
    ]:
        with winreg.OpenKeyEx(hive, key, access=winreg.KEY_READ | flags) as root_key:
            for company in enum_keys(root_key):
                if company == 'PyLauncher':
                    continue

                with winreg.OpenKey(root_key, company) as company_key:
                    for tag in enum_keys(company_key):
                        if (company, tag) in seen:
                            if company == 'PythonCore':
                                # TODO: Backwards compatibility handling
                                pass
                            continue
                        seen.add((company, tag))

                        try:
                            with winreg.OpenKey(company_key, tag + r'\InstallPath') as ip_key:
                                exec_path = get_value(ip_key, 'ExecutablePath')
                                exec_args = get_value(ip_key, 'ExecutableArguments')
                                if company == 'PythonCore' and not exec_path:
                                    # TODO: Backwards compatibility handling
                                    pass
                        except OSError:
                            exec_path, exec_args = None, None

                        if exec_path:
                            print('{}\\{} - {} {}'.format(company, tag, exec_path, exec_args or ''))
                        else:
                            print('{}\\{} - (not executable)'.format(company, tag))

This example only scans ``PythonCore`` entries for the current user. Where data
is missing, the defaults as described earlier in the PEP are substituted. Note
that these defaults are only for use under ``PythonCore``; other registrations
do not have any default values::

    # Only lists per-user PythonCore registrations
    # Uses fallback values as described in PEP 514

    import os
    import winreg

    def enum_keys(key):
        i = 0
        while True:
            try:
                yield winreg.EnumKey(key, i)
            except OSError:
                break
            i += 1

    def get_value(key, value_name):
        try:
            return winreg.QueryValue(key, value_name)
        except FileNotFoundError:
            return None

    with winreg.OpenKey(winreg.HKEY_CURRENT_USER, r"Software\Python\PythonCore") as company_key:
        print('Company:', get_value(company_key, 'DisplayName') or 'Python Software Foundation')
        print('Support:', get_value(company_key, 'SupportUrl') or 'http://www.python.org/')
        print()

        for tag in enum_keys(company_key):
            with winreg.OpenKey(company_key, tag) as tag_key:
                print('PythonCore\\' + tag)
                print('Name:', get_value(tag_key, 'DisplayName') or ('Python ' + tag))
                print('Support:', get_value(tag_key, 'SupportUrl') or 'http://www.python.org/')
                print('Version:', get_value(tag_key, 'Version') or tag[:3])
                print('SysVersion:', get_value(tag_key, 'SysVersion') or tag[:3])
                # Architecture is unknown because we are in HKCU
                # Tools may use alternate approaches to determine architecture when
                # the registration does not specify it.
                print('SysArchitecture:', get_value(tag_key, 'SysArchitecture') or '(unknown)')

            try:
                ip_key = winreg.OpenKey(company_key, tag + '\\InstallPath')
            except FileNotFoundError:
                pass
            else:
                with ip_key:
                    ip = get_value(ip_key, None)
                    exe = get_value(ip_key, 'ExecutablePath') or os.path.join(ip, 'python.exe')
                    exew = get_value(ip_key, 'WindowedExecutablePath') or os.path.join(ip, 'python.exe')
                    print('InstallPath:', ip)
                    print('ExecutablePath:', exe)
                    print('WindowedExecutablePath:', exew)
            print()

This example shows a subset of the registration that will be created by a
just-for-me install of 64-bit Python 3.6.0. Other keys may also be created::

    HKEY_CURRENT_USER\Software\Python\PythonCore
        (Default) = (value not set)
        DisplayName = "Python Software Foundation"
        SupportUrl = "http://www.python.org/"

    HKEY_CURRENT_USER\Software\Python\PythonCore\3.6
        (Default) = (value not set)
        DisplayName = "Python 3.6 (64-bit)"
        SupportUrl = "http://www.python.org/"
        Version = "3.6.0"
        SysVersion = "3.6"
        SysArchitecture = "64bit"

    HKEY_CURRENT_USER\Software\Python\PythonCore\3.6\Help\Main Python Documentation
        (Default) = "C:\Users\Me\AppData\Local\Programs\Python\Python36\Doc\python360.chm"
        DisplayName = "Python 3.6.0 Documentation"

    HKEY_CURRENT_USER\Software\Python\PythonCore\3.6\InstallPath
        (Default) = "C:\Users\Me\AppData\Local\Programs\Python\Python36\"
        ExecutablePath = "C:\Users\Me\AppData\Local\Programs\Python\Python36\python.exe"
        WindowedExecutablePath = "C:\Users\Me\AppData\Local\Programs\Python\Python36\pythonw.exe"

References
==========

.. [1] Registry Redirector (Windows)
   (https://msdn.microsoft.com/en-us/library/windows/desktop/aa384232.aspx)

Copyright
=========

This document has been placed in the public domain.
