1. Command line and environment

The Jython interpreter scans the command line and the environment for various settings.

Note: Other implementations’ command line schemes may differ. See
Alternate Implementations for further resources.

1.1. Command line

When invoking Jython, you may specify any of these options:

jython [-c command | -m module-name | script | - ] [args]

The most common use case is, of course, a simple invocation of a script:

jython myscript.py

1.1.1. Interface options

The interpreter interface resembles that of the UNIX shell, but provides some additional methods of invocation:

  • When called with standard input connected to a tty device, it prompts for commands and executes them until an EOF (an end-of-file character, you can produce that with Ctrl-D on UNIX or Ctrl-Z, Enter on Windows) is read.
  • When called with a file name argument or with a file as standard input, it reads and executes a script from that file.
  • When called with a directory name argument, it reads and executes an appropriately named script from that directory.
  • When called with -c command, it executes the Python statement(s) given as command. Here command may contain multiple statements separated by newlines. Leading whitespace is significant in Python statements!
  • When called with -m module-name, the given module is located on the Jython module path and executed as a script.

In non-interactive mode, the entire input is parsed before it is executed.

An interface option terminates the list of options consumed by the interpreter, all consecutive arguments will end up in sys.argv – note that the first element, subscript zero (sys.argv[0]), is a string reflecting the program’s source.

-c <command>

Execute the Jython code in command. command can be one ore more statements separated by newlines, with significant leading whitespace as in normal module code.

If this option is given, the first element of sys.argv will be "-c" and the current directory will be added to the start of sys.path (allowing modules in that directory to be imported as top level modules).

-m <module-name>

Search sys.path for the named module and execute its contents as the __main__ module.

Since the argument is a module name, you must not give a file extension (.py). The module-name should be a valid Jython or Python module name, but the implementation may not always enforce this (e.g. it may allow you to use a name that includes a hyphen).

If this option is given, the first element of sys.argv will be the full path to the module file. As with the -c option, the current directory will be added to the start of sys.path.

Many standard library modules contain code that is invoked on their execution as a script. An example is the timeit module:

jython -mtimeit -s ‘setup here’ ‘benchmarked code here’ jython -mtimeit -h # for details

See also:

runpy.run_module() The actual implementation of this feature.

PEP 338 – Executing modules as scripts


Execute the Python code contained in script, which must be a filesystem path (absolute or relative) referring to either a Python file, a directory containing a __main__.py file, or a zipfile containing a __main__.py file.

If this option is given, the first element of sys.argv will be the script name as given on the command line.

If the script name refers directly to a Python file, the directory containing that file is added to the start of sys.path, and the file is executed as the __main__ module.

If the script name refers to a directory or zipfile, the script name is added to the start of sys.path and the __main__.py file in that location is executed as the __main__ module.

Changed in version 2.5: Directories and zipfiles containing a __main__.py file at the top level are now considered valid Python scripts.

If no interface option is given, -i is implied, sys.argv[0] is an empty string ("") and the current directory will be added to the start of sys.path.

See also:

Invoking the Interpreter

1.1.2. Generic options

-? -h –help

Print a short description of all command line options.

Changed in version 2.5: The --help variant.

-V –version

Print the Jython version number and exit. Example output could be:

Jython 2.5.0

Changed in version 2.5: The --version variant.

1.1.3. Miscellaneous options


When a script is passed as first argument or the -c option is used, enter interactive mode after executing the script or the command, even when sys.stdin does not appear to be a terminal. The PYTHONSTARTUP file is not read.

This can be useful to inspect global variables or a stack trace when a script raises an exception. See also PYTHONINSPECT.

-Q <arg>

Division control. The argument must be one of the following:

division of int/int and long/long return an int or long (default)
new division semantics, i.e. division of int/int and long/long returns a float
old division semantics with a warning for int/int and long/long
old division semantics with a warning for all uses of the division operator

See also:

Tools/scripts/fixdiv.py for a use of warnall

PEP 238 – Changing the division operator

-S Disable the import of the module site and the site-dependent manipulations of sys.path that it entails.

Force stdin, stdout and stderr to be totally unbuffered. On systems where it matters, also put stdin, stdout and stderr in binary mode.

Note that there is internal buffering in file.readlines() and File Objects (for line in sys.stdin) which is not influenced by this option. To work around this, you will want to use file.readline() inside a while 1: loop.


-v Print a message each time a module is initialized, showing the place (filename or built-in module) from which it is loaded. When given twice (-vv), print a message for each file that is checked for when searching for a module. Also provides information on module cleanup at exit. See also PYTHONVERBOSE.
-W arg

Warning control. Jython’s warning machinery by default prints warning messages to sys.stderr. A typical warning message has the following form:

file:line: category: message

By default, each warning is printed once for each source line where it occurs. This option controls how often warnings are printed.

Multiple -W options may be given; when a warning matches more than one option, the action for the last matching option is performed. Invalid -W options are ignored (though, a warning message is printed about invalid options when the first warning is issued).

Warnings can also be controlled from within a Jython program using the warnings module.

The simplest form of argument is one of the following action strings (or a unique abbreviation):

Ignore all warnings.
Explicitly request the default behavior (printing each warning once per source line).
Print a warning each time it occurs (this may generate many messages if a warning is triggered repeatedly for the same source line, such as inside a loop).
Print each warning only only the first time it occurs in each module.
Print each warning only the first time it occurs in the program.
Raise an exception instead of printing a warning message.

The full form of argument is:


Here, action is as explained above but only applies to messages that match the remaining fields. Empty fields match all values; trailing empty fields may be omitted. The message field matches the start of the warning message printed; this match is case- insensitive. The category field matches the warning category. This must be a class name; the match test whether the actual warning category of the message is a subclass of the specified warning category. The full class name must be given. The module field matches the (fully-qualified) module name; this match is case-sensitive. The line field matches the line number, where zero matches all line numbers and is thus equivalent to an omitted line number.

See also:

warnings – the warnings module

PEP 230 – Warning framework

-jar jar

Program read from __run__.py in jar file

1.2. Environment variables

These environment variables influence Python’s behavior.


Jython installation directory


Java installation directory


Augment the default search path for module files. The format is the same as the shell’s PATH: one or more directory pathnames separated by os.pathsep (e.g. colons on Unix or semicolons on Windows). Non-existent directories are silently ignored.


Default command line arguments

1.3. Jython Launcher Options

-Jarg Pass argument through to Java VM (e.g. -J-Xmx512m)
--jdb Run under JDB
--print Print the Java command instead of executing it
--profile Run with the Java interactive Profiler (http://jiprof.sf.net)
--boot Put Jython on the boot classpath (disables bytecode verifier)

Pass remaining variables through to Jython