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 in a manner similar to how POSIX shells do it
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. The Namespace includes the [Statement][cmd2.Statement] object that was created when parsing the command line. It can be retrieved by calling cmd2_statement.get() on the Namespace.
4. Adds the usage message from the argument parser to your command's help.
5. Checks if the -h/--help option is present, and if so, displays the help message for the command

These features are all provided by the [@with_argparser][cmd2.with_argparser] decorator which is imported from cmd2.

See the argparse_example example to learn more about how to use the various cmd2 argument processing decorators in your cmd2 applications.

cmd2 provides the following decorators for assisting with parsing arguments passed to commands:

• [cmd2.decorators.with_argparser][]
• [cmd2.decorators.with_argument_list][]

All of these decorators accept an optional preserve_quotes argument which defaults to False. Setting this argument to True is useful for cases where you are passing the arguments to another command which might have its own argument parsing.

The [@with_argparser][cmd2.with_argparser] decorator can accept the following for its first argument:

1. An existing instance of argparse.ArgumentParser
2. A function or static method which returns an instance of argparse.ArgumentParser
3. Cmd or CommandSet class method which returns an instance of argparse.ArgumentParser

In all cases the @with_argparser decorator creates a deep copy of the parser instance which it stores internally. A consequence is that parsers don't need to be unique across commands.

!!! warning

Since the `@with_argparser` decorator is making a deep-copy of the parser provided, if you wish
to dynamically modify this parser at a later time, you need to retrieve this deep copy. This can
be done using `self._command_parsers.get(self.do_commandname)`.

For each command in the cmd2.Cmd subclass which requires argument parsing, create an instance of argparse.ArgumentParser() which can parse the input appropriately for the command (or provide a function/method that returns such a parser). Then decorate the command method with the @with_argparser decorator, passing the argument parser as the first parameter to the decorator. This changes the second argument of the command method, which will contain the results of ArgumentParser.parse_args().

Here's what it looks like:

from cmd2 import Cmd2ArgumentParser, with_argparser

argparser = Cmd2ArgumentParser()
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_argparser(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

`cmd2` 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.

As of the 3.0.0 release, `cmd2` sets `prog` when the instance-specific parser is created, which is later
than in previous versions.

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

!!! tip "description and epilog fields are rich objects"

While the `help` field is simply a string, both the `description` and `epilog` fields can accept any
[rich](https://github.com/Textualize/rich) renderable. This allows you to include all of rich's
built-in objects like `Text`, `Table`, and `Markdown`.

With this code:

from cmd2 import Cmd2ArgumentParser, with_argparser

argparser = Cmd2ArgumentParser()
argparser.add_argument('tag', help='tag')
argparser.add_argument('content', nargs='+', help='content to surround with tag')
@with_argparser(argparser)
def do_tag(self, args):
    """Create an HTML tag"""
    self.stdout.write('<{0}>{1}</{0}>'.format(args.tag, ' '.join(args.content)))
    self.stdout.write('\n')

the help tag command displays:

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

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

from cmd2 import Cmd2ArgumentParser, with_argparser

argparser = Cmd2ArgumentParser(description='create an HTML tag')
argparser.add_argument('tag', help='tag')
argparser.add_argument('content', nargs='+', help='content to surround with tag')
@with_argparser(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:

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:

from cmd2 import Cmd2ArgumentParser, with_argparser

argparser = Cmd2ArgumentParser(description='create an HTML tag',
                                epilog='This command cannot 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_argparser(argparser)
def do_tag(self, args):
    self.stdout.write('<{0}>{1}</{0}>'.format(args.tag, ' '.join(args.content)))
    self.stdout.write('\n')

Which yields:

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 cannot generate tags with no content, like <br/>

!!! warning

If a command **foo** is decorated with `cmd2`'s `with_argparse` decorator, then **help_foo** will not be
invoked when `help foo` is called. The [argparse](https://docs.python.org/3/library/argparse.html) module
provides a rich API which can be used to tweak every aspect of the displayed help and we encourage `cmd2`
developers to utilize that.

cmd2 has 5 different Argparse HelpFormatter classes, all of which are based on the RichHelpFormatter class from rich-argparse. The benefit is that your cmd2 applications now have more aesthetically pleasing help which includes color to make it quicker and easier to visually parse help text. This works for all supported versions of Python.

• [Cmd2HelpFormatter][cmd2.argparse_custom.Cmd2HelpFormatter] - default help formatter class
• [ArgumentDefaultsCmd2HelpFormatter][cmd2.argparse_custom.ArgumentDefaultsCmd2HelpFormatter] - adds default values to argument help
• [MetavarTypeCmd2HelpFormatter][cmd2.argparse_custom.MetavarTypeCmd2HelpFormatter] - uses the argument 'type' as the default metavar value (instead of the argument 'dest')
• [RawDescriptionCmd2HelpFormatter][cmd2.argparse_custom.RawDescriptionCmd2HelpFormatter] - retains any formatting in descriptions and epilogs
• [RawTextCmd2HelpFormatter][cmd2.argparse_custom.RawTextCmd2HelpFormatter] - retains formatting of all help text

The default Cmd2HelpFormatter class inherits from argparse.HelpFormatter. If you want a different behavior, then pass the desired class to the formatter_class argument of your argparse parser, e.g. formatter_class=ArgumentDefaultsCmd2HelpFormatter to your parser.

The default behavior of cmd2 is to pass the user input directly to your do_* methods as a string. The object passed to your method is actually a [Statement][cmd2.Statement] object, which has additional attributes that may be helpful, including arg_list and argv:

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

    def do_say(self, statement):
        # statement contains a string
        self.poutput(statement)

    def do_speak(self, statement):
        # statement also has a list of arguments
        # quoted arguments remain quoted
        for arg in statement.arg_list:
            self.poutput(arg)

    def do_articulate(self, statement):
        # statement.argv contains the command
        # and the arguments, which have had quotes
        # stripped
        for arg in statement.argv:
            self.poutput(arg)

If you don't want to access the additional attributes on the string passed to your do_* method 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][cmd2.with_argument_list] decorator to those methods that should receive an argument list instead of a string:

from cmd2 import with_argument_list

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

To pass all unknown arguments to your command as a list of strings, then decorate the command method with the @with_argparser(..., with_unknown_args=True) decorator.

Here's what it looks like:

from cmd2 import Cmd2ArgumentParser, with_argparser

dir_parser = Cmd2ArgumentParser()
dir_parser.add_argument('-l', '--long', action='store_true',
                        help="display in long format with one item per line")

@with_argparser(dir_parser, with_unknown_args=True)
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:")
        self.do_help('dir')
        self.last_result = 'Bad arguments'
        return

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

    ...

In some cases, it may be necessary to write custom argparse code that is dependent on your application's state data. To support this ability while still allowing use of the decorators, @with_argparser has an optional argument called ns_provider.

ns_provider is a Callable that accepts a cmd2.Cmd object as an argument and returns an argparse.Namespace:

Callable[[cmd2.Cmd], argparse.Namespace]

For example:

def settings_ns_provider(self) -> argparse.Namespace:
    """Populate an argparse Namespace with current settings"""
    ns = argparse.Namespace()
    ns.app_settings = self.settings
    return ns

To use this function with the @with_argparser decorator, do the following:

@with_argparser(my_parser, ns_provider=settings_ns_provider)

The Namespace is passed by the decorators to the argparse parsing functions, giving your custom code access to the state data it needs for its parsing logic.

Subcommands are supported for commands using the @with_argparser decorator. The syntax is based on argparse sub-parsers.

You may add multiple layers of subcommands for your command. cmd2 will automatically traverse and tab complete subcommands for all commands using argparse.

See the argparse_example example to learn more about how to use subcommands in your cmd2 application.

The [@as_subcommand_to][cmd2.as_subcommand_to] decorator makes adding subcommands easy.

cmd2 augments the standard argparse.nargs with range tuple capability:

• nargs=(5,) - accept 5 or more items
• nargs=(8, 12) - accept 8 to 12 items

cmd2 also provides the [Cmd2ArgumentParser][cmd2.Cmd2ArgumentParser] class which inherits from argparse.ArgumentParser and improves error and help output.

If you are using custom decorators in combination with @cmd2.with_argparser, then the order of your custom decorator(s) relative to the cmd2 decorator affects runtime behavior and argparse errors. There is nothing cmd2-specific here, this is just a side-effect of how decorators work in Python. To learn more about how decorators work, see decorator_primer.

If you want your custom decorator's runtime behavior to occur in the case of an argparse error, then that decorator needs to go after the argparse one, e.g.:

@cmd2.with_argparser(foo_parser)
@my_decorator
def do_foo(self, args: argparse.Namespace) -> None:
    """foo docs"""
    pass

However, if you do NOT want the custom decorator runtime behavior to occur during an argparse error, then that decorator needs to go before the argparse one, e.g.:

@my_decorator
@cmd2.with_argparser(bar_parser)
def do_bar(self, args: argparse.Namespace) -> None:
    """bar docs"""
    pass

The help_categories example demonstrates both above cases in a concrete fashion.

cmd2's @with_argparser decorator adds the following attributes to argparse Namespaces. To avoid naming collisions, do not use any of the names for your argparse arguments.

• cmd2_statement - cmd2.Cmd2AttributeWrapper object containing the cmd2.Statement object that was created when parsing the command line.
• cmd2_handler - cmd2.Cmd2AttributeWrapper object containing a subcommand handler function or None if one was not set.
• __subcmd_handler__ - used by cmd2 to identify the handler for a subcommand created with the @cmd2.as_subcommand_to decorator.