path: root/docs/argument_processing.rst

.. _decorators:

===================
Argument Processing
===================

``cmd2`` makes it easy to add sophisticated argument processing to your commands using the ``argparse`` python module.
``cmd2`` handles the following for you:

1. Parsing input and quoted strings like the Unix shell
2. Parse the resulting argument list using an instance of ``argparse.ArgumentParser`` that you provide
3. Passes the resulting ``argparse.Namespace`` object to your command function
4. Adds the usage message from the argument parser to your command.
5. Checks if the ``-h/--help`` option is present, and if so, display the help message for the command

These features are all provided by the ``@with_argument_parser`` decorator.

Using the argument parser decorator
===================================

For each command in the ``cmd2`` subclass which requires argument parsing,
create an instance of ``argparse.ArgumentParser()`` which can parse the
input appropriately for the command. Then decorate the command method with
the ``@with_argument_parser`` decorator, passing the argument parser as the
first parameter to the decorator. This changes the second argumen to the command method, which will contain the results
of ``ArgumentParser.parse_args()``.

Here's what it looks like::

      argparser = argparse.ArgumentParser()
      argparser.add_argument('-p', '--piglatin', action='store_true', help='atinLay')
      argparser.add_argument('-s', '--shout', action='store_true', help='N00B EMULATION MODE')
      argparser.add_argument('-r', '--repeat', type=int, help='output [n] times')
      argparser.add_argument('word', nargs='?', help='word to say')

      @with_argument_parser(argparser)
      def do_speak(self, opts)
         """Repeats what you tell me to."""
         arg = opts.word
         if opts.piglatin:
            arg = '%s%say' % (arg[1:], arg[0])
         if opts.shout:
            arg = arg.upper()
         repetitions = opts.repeat or 1
         for i in range(min(repetitions, self.maxrepeats)):
            self.poutput(arg)

.. note::

   The ``@with_argument_parser`` decorator sets the ``prog`` variable in
   the argument parser based on the name of the method it is decorating.
   This will override anything you specify in ``prog`` variable when
   creating the argument parser.


Help Messages
=============

By default, cmd2 uses the docstring of the command method when a user asks
for help on the command. When you use the ``@with_argument_parser``
decorator, the docstring for the ``do_*`` method is used to set the description for the ``argparse.ArgumentParser`` is
With this code::

   argparser = argparse.ArgumentParser()
   argparser.add_argument('tag', help='tag')
   argparser.add_argument('content', nargs='+', help='content to surround with tag')
   @with_argument_parser(argparser)
   def do_tag(self, args):
      """create a html tag"""
      self.stdout.write('<{0}>{1}</{0}>'.format(args.tag, ' '.join(args.content)))
      self.stdout.write('\n')

The ``help tag`` command displays:

.. code-block:: none

   usage: tag [-h] tag content [content ...]

   create a html tag

   positional arguments:
     tag         tag
     content     content to surround with tag

   optional arguments:
     -h, --help  show this help message and exit


If you would prefer you can set the ``description`` while instantiating the ``argparse.ArgumentParser`` and leave the
docstring on your method empty::

   argparser = argparse.ArgumentParser(description='create an html tag')
   argparser.add_argument('tag', help='tag')
   argparser.add_argument('content', nargs='+', help='content to surround with tag')
   @with_argument_parser(argparser)
   def do_tag(self, args):
      self.stdout.write('<{0}>{1}</{0}>'.format(args.tag, ' '.join(args.content)))
      self.stdout.write('\n')

Now when the user enters ``help tag`` they see:

.. code-block:: none

   usage: tag [-h] tag content [content ...]

   create an html tag

   positional arguments:
     tag         tag
     content     content to surround with tag

   optional arguments:
     -h, --help  show this help message and exit


To add additional text to the end of the generated help message, use the ``epilog`` variable::

   argparser = argparse.ArgumentParser(
      description='create an html tag',
      epilog='This command can not generate tags with no content, like <br/>.'
   )
   argparser.add_argument('tag', help='tag')
   argparser.add_argument('content', nargs='+', help='content to surround with tag')
   @with_argument_parser(argparser)
   def do_tag(self, args):
      self.stdout.write('<{0}>{1}</{0}>'.format(args.tag, ' '.join(args.content)))
      self.stdout.write('\n')

Which yields:

.. code-block:: none

   usage: tag [-h] tag content [content ...]

   create an html tag

   positional arguments:
     tag         tag
     content     content to surround with tag

   optional arguments:
     -h, --help  show this help message and exit

   This command can not generate tags with no content, like <br/>


Receiving an argument list
==========================

The default behavior of ``cmd2`` is to pass the user input directly to your
``do_*`` methods as a string. If you don't want to use the full argument parser support outlined above, you can still have ``cmd2`` apply shell parsing rules to the user input and pass you a list of arguments instead of a string. Apply the ``@with_argument_list`` decorator to those methods that should receive an argument list instead of a string::

   class CmdLineApp(cmd2.Cmd):
      """ Example cmd2 application. """

      def do_say(self, cmdline):
         # cmdline contains a string
         pass

      @with_argument_list
      def do_speak(self, arglist):
         # arglist contains a list of arguments
         pass


Using the argument parser decorator and also receiving a a list of unknown positional arguments
===============================================================================================
If you want all unknown arguments to be passed to your command as a list of strings, then
decorate the command method with the ``@with_argparser_and_list`` decorator.

Here's what it looks like::

    dir_parser = argparse.ArgumentParser()
    dir_parser.add_argument('-l', '--long', action='store_true', help="display in long format with one item per line")

    @with_argparser_and_list(dir_parser)
    def do_dir(self, args, unknown):
        """List contents of current directory."""
        # No arguments for this command
        if unknown:
            self.perror("dir does not take any positional arguments:", traceback_war=False)
            self.do_help('dir')
            self._last_result = CmdResult('', 'Bad arguments')
            return

        # Get the contents as a list
        contents = os.listdir(self.cwd)

        ...

Deprecated optparse support
===========================

The ``optparse`` library has been deprecated since Python 2.7 (released on July
3rd 2010) and Python 3.2 (released on February 20th, 2011). ``optparse`` is
still included in the python standard library, but the documentation
recommends using ``argparse`` instead.

``cmd2`` includes a decorator which can parse arguments using ``optparse``. This decorator is deprecated just like the ``optparse`` library.

Here's an example::

   opts = [make_option('-p', '--piglatin', action="store_true", help="atinLay"),
           make_option('-s', '--shout', action="store_true", help="N00B EMULATION MODE"),
           make_option('-r', '--repeat', type="int", help="output [n] times")]

   @options(opts, arg_desc='(text to say)')
   def do_speak(self, arg, opts=None):
     """Repeats what you tell me to."""
     arg = ''.join(arg)
     if opts.piglatin:
         arg = '%s%say' % (arg[1:], arg[0])
     if opts.shout:
         arg = arg.upper()
     repetitions = opts.repeat or 1
     for i in range(min(repetitions, self.maxrepeats)):
         self.poutput(arg)


The optparse decorator performs the following key functions for you:

1. Use `shlex` to split the arguments entered by the user.
2. Parse the arguments using the given optparse options.
3. Replace the `__doc__` string of the decorated function (i.e. do_speak) with the help string generated by optparse.
4. Call the decorated function (i.e. do_speak) passing an additional parameter which contains the parsed options.