CLI subcommands with precise help output

Introduction#

Different ways to create subcommands like in hg or svn with the exact command line interface and help output as shown in Remarks section.

https://stackoverflow.com/documentation/python/1382/parsing-command-line-arguments covers broader topic of arguments parsing.

Remarks#

Different ways to create subcommands like in hg or svn with the command line interface shown in the help message:

usage: sub <command>

commands:

  status -  show status
  list   -  print list

Native way (no libraries)

"""
usage: sub <command>

commands:

  status -  show status
  list   -  print list
"""

import sys

def check():
    print("status")
    return 0

if sys.argv[1:] == ['status']:
    sys.exit(check())
elif sys.argv[1:] == ['list']:
    print("list")
else:
    print(__doc__.strip())

Output without arguments:

usage: sub <command>

commands:

  status -  show status
  list   -  print list

Pros:

no deps
everybody should be able to read that
complete control over help formatting

argparse (default help formatter)

import argparse
import sys

def check():
    print("status")
    return 0

parser = argparse.ArgumentParser(prog="sub", add_help=False)
subparser = parser.add_subparsers(dest="cmd")

subparser.add_parser('status', help='show status')
subparser.add_parser('list', help='print list')

# hack to show help when no arguments supplied
if len(sys.argv) == 1:
    parser.print_help()
    sys.exit(0)

args = parser.parse_args()

if args.cmd == 'list':
    print('list')
elif args.cmd == 'status':
    sys.exit(check())

Output without arguments:

usage: sub {status,list} ...

positional arguments:
  {status,list}
    status       show status
    list         print list

Pros:

comes with Python
option parsing is included

argparse (custom help formatter)

Extended version of https://stackoverflow.com/documentation/python/7701/cli-subcommands/25282/argparse-default-help-formatter that fixed help output.

import argparse
import sys

class CustomHelpFormatter(argparse.HelpFormatter):
    def _format_action(self, action):
        if type(action) == argparse._SubParsersAction:
            # inject new class variable for subcommand formatting
            subactions = action._get_subactions()
            invocations = [self._format_action_invocation(a) for a in subactions]
            self._subcommand_max_length = max(len(i) for i in invocations)

        if type(action) == argparse._SubParsersAction._ChoicesPseudoAction:
            # format subcommand help line
            subcommand = self._format_action_invocation(action) # type: str
            width = self._subcommand_max_length
            help_text = ""
            if action.help:
                help_text = self._expand_help(action)
            return "  {:{width}} -  {}\n".format(subcommand, help_text, width=width)

        elif type(action) == argparse._SubParsersAction:
            # process subcommand help section
            msg = '\n'
            for subaction in action._get_subactions():
                msg += self._format_action(subaction)
            return msg
        else:
            return super(CustomHelpFormatter, self)._format_action(action)


def check():
    print("status")
    return 0

parser = argparse.ArgumentParser(usage="sub <command>", add_help=False,
             formatter_class=CustomHelpFormatter)

subparser = parser.add_subparsers(dest="cmd")
subparser.add_parser('status', help='show status')
subparser.add_parser('list', help='print list')

# custom help messge
parser._positionals.title = "commands"

# hack to show help when no arguments supplied
if len(sys.argv) == 1:
    parser.print_help()
    sys.exit(0)

args = parser.parse_args()

if args.cmd == 'list':
    print('list')
elif args.cmd == 'status':
    sys.exit(check())

Output without arguments:

usage: sub <command>

commands:

  status -  show status
  list   -  print list