Add PEP 514: Python registration in the Windows registry
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Steve Dower
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PEP: 514
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Title: Python registration in the Windows registry
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Version: $Revision$
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Last-Modified: $Date$
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Author: Steve Dower <steve.dower@python.org>
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Status: Draft
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Type: Informational
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Content-Type: text/x-rst
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Created: 02-Feb-2016
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Post-History: 02-Feb-2016
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Abstract
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========
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This PEP defines a schema for the Python registry key to allow third-party
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installers to register their installation, and to allow applications to detect
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and correctly display all Python environments on a user's machine. No
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implementation changes to Python are proposed with this PEP.
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Python environments are not required to be registered unless they want to be
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automatically discoverable by external tools.
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The schema matches the registry values that have been used by the official
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installer since at least Python 2.5, and the resolution behaviour matches the
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behaviour of the official Python releases.
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Motivation
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==========
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When installed on Windows, the official Python installer creates a registry key
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for discovery and detection by other applications. This allows tools such as
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installers or IDEs to automatically detect and display a user's Python
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installations.
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Third-party installers, such as those used by distributions, typically create
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identical keys for the same purpose. Most tools that use the registry to detect
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Python installations only inspect the keys used by the official installer. As a
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result, third-party installations that wish to be discoverable will overwrite
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these values, resulting in users "losing" their Python installation.
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By describing a layout for registry keys that allows third-party installations
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to register themselves uniquely, as well as providing tool developers guidance
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for discovering all available Python installations, these collisions should be
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prevented.
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Definitions
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===========
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A "registry key" is the equivalent of a file-system path into the registry. Each
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key may contain "subkeys" (keys nested within keys) and "values" (named and
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typed attributes attached to a key).
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``HKEY_CURRENT_USER`` is the root of settings for the currently logged-in user,
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and this user can generally read and write all settings under this root.
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``HKEY_LOCAL_MACHINE`` is the root of settings for all users. Generally, any
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user can read these settings but only administrators can modify them. It is
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typical for values under ``HKEY_CURRENT_USER`` to take precedence over those in
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``HKEY_LOCAL_MACHINE``.
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On 64-bit Windows, ``HKEY_LOCAL_MACHINE\Software\Wow6432Node`` is a special key
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that 32-bit processes transparently read and write to rather than accessing the
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``Software`` key directly.
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Structure
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=========
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We consider there to be a single collection of Python environments on a machine,
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where the collection may be different for each user of the machine. There are
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three potential registry locations where the collection may be stored based on
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the installation options of each environment::
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HKEY_CURRENT_USER\Software\Python\<Company>\<Tag>
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HKEY_LOCAL_MACHINE\Software\Python\<Company>\<Tag>
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HKEY_LOCAL_MACHINE\Software\Wow6432Node\Python\<Company>\<Tag>
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Environments are uniquely identified by their Company-Tag pair, with two options
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for conflict resolution: include everything, or give priority to user
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preferences.
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Tools that include every installed environment, even where the Company-Tag pairs
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match, should ensure users can easily identify whether the registration was
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per-user or per-machine.
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Tools that give priority to user preferences must ignore values from
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``HKEY_LOCAL_MACHINE`` when a matching Company-Tag pair exists is in
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``HKEY_CURRENT_USER``.
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Official Python releases use ``PythonCore`` for Company, and the value of
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``sys.winver`` for Tag. Other registered environments may use any values for
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Company and Tag. Recommendations are made in the following sections.
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Python environments are not required to register themselves unless they want to
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be automatically discoverable by external tools.
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Backwards Compatibility
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-----------------------
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Python 3.4 and earlier did not distinguish between 32-bit and 64-bit builds in
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``sys.winver``. As a result, it is possible to have valid side-by-side
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installations of both 32-bit and 64-bit interpreters.
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To ensure backwards compatibility, applications should treat environments listed
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under the following two registry keys as distinct, even when the Tag matches::
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HKEY_LOCAL_MACHINE\Software\Python\PythonCore\<Tag>
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HKEY_LOCAL_MACHINE\Software\Wow6432Node\Python\PythonCore\<Tag>
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Environments listed under ``HKEY_CURRENT_USER`` may be treated as distinct from
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both of the above keys, potentially resulting in three environments discovered
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using the same Tag. Alternatively, a tool may determine whether the per-user
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environment is 64-bit or 32-bit and give it priority over the per-machine
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environment, resulting in a maximum of two discovered environments.
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It is not possible to detect side-by-side installations of both 64-bit and
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32-bit versions of Python prior to 3.5 when they have been installed for the
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current user. Python 3.5 and later always uses different Tags for 64-bit and
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32-bit versions.
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Environments registered under other Company names must use distinct Tags to
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support side-by-side installations. There is no backwards compatibility
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allowance.
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Company
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-------
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The Company part of the key is intended to group related environments and to
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ensure that Tags are namespaced appropriately. The key name should be
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alphanumeric without spaces and likely to be unique. For example, a trademarked
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name, a UUID, or a hostname would be appropriate::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp
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HKEY_CURRENT_USER\Software\Python\6C465E66-5A8C-4942-9E6A-D29159480C60
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HKEY_CURRENT_USER\Software\Python\www.example.com
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The company name ``PyLauncher`` is reserved for the PEP 397 launcher
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(``py.exe``). It does not follow this convention and should be ignored by tools.
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If a string value named ``DisplayName`` exists, it should be used to identify
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the environment category to users. Otherwise, the name of the key should be
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used.
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If a string value named ``SupportUrl`` exists, it may be displayed or otherwise
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used to direct users to a web site related to the environment.
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A complete example may look like::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp
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(Default) = (value not set)
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DisplayName = "Example Corp"
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SupportUrl = "http://www.example.com"
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Tag
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---
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The Tag part of the key is intended to uniquely identify an environment within
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those provided by a single company. The key name should be alphanumeric without
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spaces and stable across installations. For example, the Python language
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version, a UUID or a partial/complete hash would be appropriate; an integer
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counter that increases for each new environment may not::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp\3.6
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HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66
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If a string value named ``DisplayName`` exists, it should be used to identify
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the environment to users. Otherwise, the name of the key should be used.
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If a string value named ``SupportUrl`` exists, it may be displayed or otherwise
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used to direct users to a web site related to the environment.
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If a string value named ``Version`` exists, it should be used to identify the
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version of the environment. This is independent from the version of Python
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implemented by the environment.
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If a string value named ``SysVersion`` exists, it must be in ``x.y`` or
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``x.y.z`` format matching the version returned by ``sys.version_info`` in the
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interpreter. Otherwise, if the Tag matches this format it is used. If not, the
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Python version is unknown.
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Note that each of these values is recommended, but optional. A complete example
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may look like this::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66
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(Default) = (value not set)
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DisplayName = "Distro 3"
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SupportUrl = "http://www.example.com/distro-3"
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Version = "3.0.12345.0"
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SysVersion = "3.6.0"
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InstallPath
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-----------
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Beneath the environment key, an ``InstallPath`` key must be created. This key is
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always named ``InstallPath``, and the default value must match ``sys.prefix``::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp\3.6\InstallPath
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(Default) = "C:\ExampleCorpPy36"
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If a string value named ``ExecutablePath`` exists, it must be a path to the
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``python.exe`` (or equivalent) executable. Otherwise, the interpreter executable
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is assumed to be called ``python.exe`` and exist in the directory referenced by
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the default value.
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If a string value named ``WindowedExecutablePath`` exists, it must be a path to
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the ``pythonw.exe`` (or equivalent) executable. Otherwise, the windowed
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interpreter executable is assumed to be called ``pythonw.exe`` and exist in the
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directory referenced by the default value.
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A complete example may look like::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66\InstallPath
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(Default) = "C:\ExampleDistro30"
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ExecutablePath = "C:\ExampleDistro30\ex_python.exe"
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WindowedExecutablePath = "C:\ExampleDistro30\ex_pythonw.exe"
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Help
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----
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Beneath the environment key, a ``Help`` key may be created. This key is always
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named ``Help`` if present and has no default value.
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Each subkey of ``Help`` specifies a documentation file, tool, or URL associated
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with the environment. The subkey may have any name, and the default value is a
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string appropriate for passing to ``os.startfile`` or equivalent.
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If a string value named ``DisplayName`` exists, it should be used to identify
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the help file to users. Otherwise, the key name should be used.
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A complete example may look like::
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HKEY_CURRENT_USER\Software\Python\ExampleCorp\6C465E66\Help
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Python\
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(Default) = "C:\ExampleDistro30\python36.chm"
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DisplayName = "Python Documentation"
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Extras\
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(Default) = "http://www.example.com/tutorial"
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DisplayName = "Example Distro Online Tutorial"
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Other Keys
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----------
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Some other registry keys are used for defining or inferring search paths under
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certain conditions. A third-party installation is permitted to define these keys
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under their Company-Tag key, however, the interpreter must be modified and
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rebuilt in order to read these values.
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Copyright
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=========
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This document has been placed in the public domain.
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