python-peps/pep-0514.txt

PEP: 514
Title: Python registration in the Windows registry
Version: $Revision$
Last-Modified: $Date$
Author: Steve Dower <steve.dower@python.org>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 02-Feb-2016
Post-History: 02-Feb-2016, 01-Mar-2016

Abstract
========

This PEP defines a schema for the Python registry key to allow third-party
installers to register their installation, and to allow 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.

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.

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.

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, resulting in users "losing" their 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.

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).

``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.

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>

Environments are uniquely identified by their Company-Tag pair, with two options
for conflict resolution: include everything, or give priority to user
preferences.

Tools that include every installed environment, even where the Company-Tag pairs
match, should ensure users can easily identify whether the registration was
per-user or per-machine.

When tools are selecting a single installed environment from all registered
environments, the intent is that user preferences from ``HKEY_CURRENT_USER``
will override matching Company-Tag pairs in ``HKEY_LOCAL_MACHINE``.

Official Python releases use ``PythonCore`` for Company, and the value of
``sys.winver`` for Tag. Other registered environments may use any values for
Company and Tag. Recommendations are made in the following sections.

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 possible to have valid side-by-side
installations of both 32-bit and 64-bit interpreters.

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.

Environments registered under other Company names 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, a UUID, or a hostname would be appropriate::

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

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 category to users. Otherwise, the name of the key should be
used.

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.

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; an integer
counter that increases for each new environment may not::

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

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.

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.

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.

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. Otherwise, if the Tag matches this format it is used. If not, the
Python version is unknown.

Note that each of these values is recommended, but optional. A complete example
may look like this::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66
        (Default) = (value not set)
        DisplayName = "Distro 3"
        SupportUrl = "http://www.example.com/distro-3"
        Version = "3.0.12345.0"
        SysVersion = "3.6.0"

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 a path to the
``python.exe`` (or equivalent) executable. Otherwise, the interpreter executable
is assumed to be called ``python.exe`` and exist in the directory referenced by
the default value.

If a string value named ``WindowedExecutablePath`` exists, it must be a path to
the ``pythonw.exe`` (or equivalent) executable. Otherwise, the windowed
interpreter executable is assumed to be called ``pythonw.exe`` and exist in the
directory referenced by the default value.

A complete example may look like::

    HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66\InstallPath
        (Default) = "C:\ExampleDistro30"
        ExecutablePath = "C:\ExampleDistro30\ex_python.exe"
        WindowedExecutablePath = "C:\ExampleDistro30\ex_pythonw.exe"

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
----------

Some other registry keys are used for defining or inferring search paths under
certain conditions. A third-party installation is permitted to define these keys
under their Company-Tag key, however, the interpreter must be modified and
rebuilt in order to read these values. Alternatively, the interpreter may be
modified to not use any registry keys for determining search paths. Making such
changes is a decision for the third party; this PEP makes no recommendation 
either way.

Copyright
=========

This document has been placed in the public domain.