# `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/db.py
    from tornado.options import define, options

    define("mysql_host", default="127.0.0.1:3306", help="Main user DB")
    define("memcache_hosts", default="127.0.0.1:11011", multiple=True,
           help="Main user memcache servers")

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

    # myapp/server.py
    from tornado.options import define, options

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

    def start_server():
        app = make_app()
        app.listen(options.port)

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

    if __name__ == '__main__':
        tornado.options.parse_command_line()
        # or
        tornado.options.parse_config_file("/etc/server.conf")

Note

When using multiple `parse_*` functions, pass `final=False` to all but the last one, or side effects may occur twice (in particular, this can result in log messages being doubled).

`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.

Note

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
    parse_command_line()

Note

`parse_command_line` or `parse_config_file` function should called after logging configuration and user-defined command line flags using the `callback` option definition, or these configurations will not take effect.

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: str, default: Any = None, type: type | None = None, help: str | None = None, metavar: str | None = None, multiple: bool = False, group: str | None = None, callback: Callable[[Any], None] | None = None) → None

  Defines an option in the global namespace.

  See `OptionParser.define`.

tornado.options.options

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

tornado.options.parse_command_line(args: List[str] | None = None, final: bool = True) → List[str]

  Parses global options from the command line.

  See `OptionParser.parse_command_line`.

tornado.options.parse_config_file(path: str, final: bool = True) → None

  Parses global options from a config file.

  See `OptionParser.parse_config_file`.

tornado.options.print_help(file=sys.stderr)

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

  See `OptionParser.print_help`.

tornado.options.add_parse_callback(callback: Callable[[], None]) → None

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

  See `OptionParser.add_parse_callback`

exception tornado.options.Error

  Exception raised by errors in the options module.

## OptionParser class

class tornado.options.OptionParser

  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: str, default: Any = None, type: type | None = None, help: str | None = None, metavar: str | None = None, multiple: bool = False, group: str | None = None, callback: Callable[[Any], None] | None = None) → None

  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: List[str] | None = None, final: bool = True) → List[str]

  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: str, final: bool = True) → None

  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 may either be the specified type for the option or strings (in which case they will be parsed the same way as in `parse_command_line`)

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

      port = 80
      mysql_host = 'mydb.example.com:3306'
      # Both lists and comma-separated strings are allowed for
      # multiple=True.
      memcache_hosts = ['cache1.example.com:11011',
                        'cache2.example.com:11011']
      memcache_hosts = 'cache1.example.com:11011,cache2.example.com:11011'

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

  Note

  `tornado.options` is primarily a command-line library. Config file support is provided for applications that wish to use it, but applications that prefer config files may wish to look at other libraries instead.

  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.

  Changed in version 5.1: Added the ability to set options via strings in config files.

OptionParser.print_help(file: TextIO | None = None) → None

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

OptionParser.add_parse_callback(callback: Callable[[], None]) → None

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

OptionParser.mockable() → _Mockable

  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 options.name == value

OptionParser.items() → Iterable[Tuple[str, Any]]

  An iterable of (name, value) pairs.

  Added in version 3.1.

OptionParser.as_dict() → Dict[str, Any]

  The names and values of all options.

  Added in version 3.1.

OptionParser.groups() → Set[str]

  The set of option-groups created by `define`.

  Added in version 3.1.

OptionParser.group_dict(group: str) → Dict[str, Any]

  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')

      parse_command_line()

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

  Added in version 3.1.