The Jython Registry

Because there is no good platform independent equivalent of the Windows Registry (or Unix envrionment variables) Java has it's own environment variable namespace. Jython aquires it's namespace from the following three sources (later sources override defaults found in earlier places).

The Java system properties: typically passed in on the command line as options to the java interpreter.
The Jython "registry" file, which contains prop=value pairs. See below for the algorithm Jython uses to find the registry file.
The user's personal registry file, which contains similarly formated prop/value pairs. The user's registry file is at "user.home"+"/.jython"
Jython properties: Specified on the command line as options to the jython class. See the -D option to the interpreter.

Registry Properties

The following properties are recognized by Jython. There may be others that aren't documented here; consult the comments in registry file for details.

python.path: Equivalent to CPython's PYTHONPATH environment variable
python.cachedir: The directory to use for caches - currently just package information. This directory must be writable by the user. If the directory is an absolute path, it is used as given, otherwise it is interpreted as relative to sys.prefix.
python.verbose: Sets the verbosity level for varying degrees of informative messages. Valid values in order of increasing verbosity are "error", "warning", "message", "comment", "debug"
python.security.respectJavaAccessibility: Normally, Jython can only provide access to public members of classes. However if this property is set to false and you are using Java 1.2 then Jython can access non-public fields, methods, and constructors.
python.jythonc.compiler: The Java compiler to use with the jythonc tool, which now generates Java source code. This should be the absolute path to a Java compiler, or the name of a compiler on your standard PATH.
python.jythonc.classpath: Extensions to the standard java.class.path property for use with jythonc. This is useful if you use Jikes as your compiler.
python.jythonc.compileropts: Options to pass to the Java compiler when using jythonc.
python.console: The name of a console class. An alternative console class that supports GNU readline can be installed with this property. Jython already include such a console class and it can be enabled by setting this property to org.python.util.ReadlineConsole
python.console.readlinelib: Allow a choice of backing implementation for GNU readline support. Can be either GnuReadline or Editline. This property is only used when python.console is set to org.python.util.ReadlineConsole.

Finding the Registry File

The following steps are used to find the Jython registry file, and also to set the Python values for sys.prefix. First a root directory is calculated:

If there is a property called python.home, this is used as the root directory.
Otherwise, the property install.root is used if it exists.
If neither of those properties exist, then Jython searches for the file "jython.jar" on the Java classpath, as defined in the system property java.class.path. The actual file system isn't searched, only the paths defined on the classpath (one of them must literally include "jython.jar").

Once the root directory is found, sys.prefix and sys.exec_prefix are set to this, and sys.path has rootdir/Lib appended to it. The registry file used is then rootdir/registry.