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.