tornado.options — Command-line parsing

A command line parsing module that lets modules define their own options.

Each module defines its own options which are added to the global option namespace, e.g.:

from tornado.options import define, options

define("mysql_host", default="", help="Main user DB")
define("memcache_hosts", default="", multiple=True,
       help="Main user memcache servers")

def connect():
    db = database.Connection(options.mysql_host)

The main() method of your application does not need to be aware of all of the options used throughout your program; they are all automatically loaded when the modules are loaded. However, all modules that define options must have been imported before the command line is parsed.

Your main() method can parse the command line or parse a config file with either:

# or

Command line formats are what you would expect (--myoption=myvalue). Config files are just Python files. Global names become options, e.g.:

myoption = "myvalue"
myotheroption = "myothervalue"

We support datetimes, timedeltas, ints, and floats (just pass a type kwarg to define). We also accept multi-value options. See the documentation for define() below.

tornado.options.options is a singleton instance of OptionParser, and the top-level functions in this module (define, parse_command_line, etc) simply call methods on it. You may create additional OptionParser instances to define isolated sets of options, such as for subcommands.


By default, several options are defined that will configure the standard logging module when parse_command_line or parse_config_file are called. If you want Tornado to leave the logging configuration alone so you can manage it yourself, either pass --logging=none on the command line or do the following to disable it in code:

from tornado.options import options, parse_command_line
options.logging = None

Changed in version 4.3: Dashes and underscores are fully interchangeable in option names; options can be defined, set, and read with any mix of the two. Dashes are typical for command-line usage while config files require underscores.

Global functions

tornado.options.define(name, default=None, type=None, help=None, metavar=None, multiple=False, group=None, callback=None)[source]

Defines an option in the global namespace.

See OptionParser.define.


Global options object. All defined options are available as attributes on this object.

tornado.options.parse_command_line(args=None, final=True)[source]

Parses global options from the command line.

See OptionParser.parse_command_line.

tornado.options.parse_config_file(path, final=True)[source]

Parses global options from a config file.

See OptionParser.parse_config_file.


Prints all the command line options to stderr (or another file).

See OptionParser.print_help.


Adds a parse callback, to be invoked when option parsing is done.

See OptionParser.add_parse_callback

exception tornado.options.Error[source]

Exception raised by errors in the options module.

OptionParser class

class tornado.options.OptionParser[source]

A collection of options, a dictionary with object-like access.

Normally accessed via static functions in the tornado.options module, which reference a global instance.


A sequence of (name, value) pairs.

New in version 3.1.


The set of option-groups created by define.

New in version 3.1.


The names and values of options in a group.

Useful for copying options into Application settings:

from tornado.options import define, parse_command_line, options

define('template_path', group='application')
define('static_path', group='application')


application = Application(
    handlers, **options.group_dict('application'))

New in version 3.1.


The names and values of all options.

New in version 3.1.

define(name, default=None, type=None, help=None, metavar=None, multiple=False, group=None, callback=None)[source]

Defines a new command line option.

If type is given (one of str, float, int, datetime, or timedelta) or can be inferred from the default, we parse the command line arguments based on the given type. If multiple is True, we accept comma-separated values, and the option value is always a list.

For multi-value integers, we also accept the syntax x:y, which turns into range(x, y) - very useful for long integer ranges.

help and metavar are used to construct the automatically generated command line help string. The help message is formatted like:

--name=METAVAR      help string

group is used to group the defined options in logical groups. By default, command line options are grouped by the file in which they are defined.

Command line option names must be unique globally. They can be parsed from the command line with parse_command_line or parsed from a config file with parse_config_file.

If a callback is given, it will be run with the new value whenever the option is changed. This can be used to combine command-line and file-based options:

define("config", type=str, help="path to config file",
       callback=lambda path: parse_config_file(path, final=False))

With this definition, options in the file specified by --config will override options set earlier on the command line, but can be overridden by later flags.

parse_command_line(args=None, final=True)[source]

Parses all options given on the command line (defaults to sys.argv).

Note that args[0] is ignored since it is the program name in sys.argv.

We return a list of all arguments that are not parsed as options.

If final is False, parse callbacks will not be run. This is useful for applications that wish to combine configurations from multiple sources.

parse_config_file(path, final=True)[source]

Parses and loads the Python config file at the given path.

If final is False, parse callbacks will not be run. This is useful for applications that wish to combine configurations from multiple sources.

Changed in version 4.1: Config files are now always interpreted as utf-8 instead of the system default encoding.


Prints all the command line options to stderr (or another file).


Adds a parse callback, to be invoked when option parsing is done.


Returns a wrapper around self that is compatible with mock.patch.

The mock.patch function (included in the standard library unittest.mock package since Python 3.3, or in the third-party mock package for older versions of Python) is incompatible with objects like options that override __getattr__ and __setattr__. This function returns an object that can be used with mock.patch.object to modify option values:

with mock.patch.object(options.mockable(), 'name', value):
    assert == value