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tornado.options — Command-line parsing

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

This module is inspired by Google’s gflags. The primary difference with libraries such as argparse is that a global registry is used so that options may be defined in any module (it also enables tornado.log by default). The rest of Tornado does not depend on this module, so feel free to use argparse or other configuration libraries if you prefer them.

Options must be defined with tornado.options.define before use, generally at the top level of a module. The options are then accessible as attributes of tornado.options.options:

# myapp/
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)

# myapp/
from tornado.options import define, options

define("port", default=8080, help="port to listen on")

def start_server():
    app = make_app()

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 parse_command_line or parse_config_file:

import myapp.db, myapp.server
import tornado.options

if __name__ == '__main__':
    # or

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.

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.

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

Defines a new command line option.

type can be any of str, int, float, bool, datetime, or timedelta. If no type is given but a default is, type is the type of default. Otherwise, type defaults to str.

If multiple is True, the option value is a list of type instead of an instance of type.

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.

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.

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

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

Options look like --option=value and are parsed according to their type. For boolean options, --option is equivalent to --option=true

If the option has multiple=True, comma-separated values are accepted. For multi-value integer options, the syntax x:y is also accepted and equivalent to range(x, y).

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.

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

Parses and loads the config file at the given path.

The config file contains Python code that will be executed (so it is not safe to use untrusted config files). Anything in the global namespace that matches a defined option will be used to set that option’s value.

Options are not parsed from strings as they would be on the command line; they should be set to the correct type (this means if you have datetime or timedelta options you will need to import those modules in the config file.

Example (using the options defined in the top-level docs of this module):

port = 80
mysql_host = ''
memcache_hosts = ['',

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.

Changed in version 4.4: The special variable __file__ is available inside config files, specifying the absolute path to the config file itself.


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

A sequence of (name, value) pairs.

New in version 3.1.


The names and values of all options.

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.