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. The Namespace includes the 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.
  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_argparser decorator which is importable from cmd2.

See the either the argprint or decorator 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:

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.

Argument Parsing

For each command in the cmd2 subclass which requires argument parsing, create a unique instance of argparse.ArgumentParser() which can parse the input appropriately for the command. 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 to the command method, which will contain the results of ArgumentParser.parse_args().

Here’s what it looks like:

import argparse
from cmd2 import with_argparser

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_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)

Warning

It is important that each command which uses the @with_argparser decorator be passed a unique instance of a parser. This limitation is due to bugs in CPython prior to Python 3.7 which make it impossible to make a deep copy of an instance of a argparse.ArgumentParser.

See the table_display example for a work-around that demonstrates how to create a function which returns a unique instance of the parser you want.

Note

The @with_argparser 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_argparser decorator, the docstring for the do_* method is used to set the description for the argparse.ArgumentParser.

With this code:

import argparse
from cmd2 import with_argparser

argparser = argparse.ArgumentParser()
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 a 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 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:

import argparse
from cmd2 import with_argparser

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_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:

import argparse
from cmd2 import with_argparser

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

Warning

If a command foo is decorated with one of cmd2’s argparse decorators, then help_foo will not be invoked when help foo is called. The argparse 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.

Argument List

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 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 you``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 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

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_unknown_args decorator.

Here’s what it looks like:

import argparse
from cmd2 import with_argparser_and_unknown_args

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(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 = CommandResult('', 'Bad arguments')
        return

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

    ...

Using A Custom Namespace

In some cases, it may be necessary to write custom argparse code that is dependent on state data of your application. To support this ability while still allowing use of the decorators, both @with_argparser and @with_argparser_and_unknown_args have 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 argparse decorators, do the following:

@with_argparser(my_parser, ns_provider=settings_ns_provider)

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

Subcommands

Subcommands are supported for commands using either the @with_argparser or @with_argparser_and_unknown_args decorator. The syntax for supporting them 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 subcommands example to learn more about how to use subcommands in your cmd2 application.

Argparse Extensions

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 cmd2.argparse_custom.Cmd2ArgumentParser class which inherits from argparse.ArgumentParser and improves error and help output.

Decorator Order

If you are using custom decorators in combination with either @cmd2.with_argparser or @cmd2.with_argparser_and_unknown_args, then the order of your custom decorator(s) relative to the cmd2 decorator matters when it comes to 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 even in the case of an argparse error, then that decorator needs to go before the arpgarse 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.

Reserved Argument Names

cmd2 argparse decorators add 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 cmd2.Statement object that was created when parsing the command line.
  • __statement__ - cmd2.Statement object that was created when parsing the command line. (This is deprecated and will be removed in 2.0.0.) Use cmd2_statement instead.
  • __subcmd_handler__ - used by cmd2 to identify the handler for a subcommand created with @cmd2.as_subcommand_to decorator.
  • cmd2_handler - cmd2.Cmd2AttributeWrapper object containing a subcommand handler function or None if one was not set.