argparse subparser monolithic help output

Here is complete soulution with custom help handler (almost all code from @Adaephon answer):

import argparse


class _HelpAction(argparse._HelpAction):

    def __call__(self, parser, namespace, values, option_string=None):
        parser.print_help()

        # retrieve subparsers from parser
        subparsers_actions = [
            action for action in parser._actions
            if isinstance(action, argparse._SubParsersAction)]
        # there will probably only be one subparser_action,
        # but better save than sorry
        for subparsers_action in subparsers_actions:
            # get all subparsers and print help
            for choice, subparser in subparsers_action.choices.items():
                print("Subparser '{}'".format(choice))
                print(subparser.format_help())

        parser.exit()

# create the top-level parser
parser = argparse.ArgumentParser(prog='PROG', add_help=False)  # here we turn off default help action

parser.add_argument('--help', action=_HelpAction, help='help for help if you need some help')  # add custom help

parser.add_argument('--foo', action='store_true', help='foo help')
subparsers = parser.add_subparsers(help='sub-command help')

# create the parser for the "a" command
parser_a = subparsers.add_parser('a', help='a help')
parser_a.add_argument('bar', type=int, help='bar help')

# create the parser for the "b" command
parser_b = subparsers.add_parser('b', help='b help')
parser_b.add_argument('--baz', choices='XYZ', help='baz help')

parsed_args = parser.parse_args()

This is a bit tricky, as argparse does not expose a list of defined sub-parsers directly. But it can be done:

import argparse

# create the top-level parser
parser = argparse.ArgumentParser(prog='PROG')
parser.add_argument('--foo', action='store_true', help='foo help')
subparsers = parser.add_subparsers(help='sub-command help')

# create the parser for the "a" command
parser_a = subparsers.add_parser('a', help='a help')
parser_a.add_argument('bar', type=int, help='bar help')

# create the parser for the "b" command
parser_b = subparsers.add_parser('b', help='b help')
parser_b.add_argument('--baz', choices='XYZ', help='baz help')
# print main help
print(parser.format_help())

# retrieve subparsers from parser
subparsers_actions = [
    action for action in parser._actions 
    if isinstance(action, argparse._SubParsersAction)]
# there will probably only be one subparser_action,
# but better safe than sorry
for subparsers_action in subparsers_actions:
    # get all subparsers and print help
    for choice, subparser in subparsers_action.choices.items():
        print("Subparser '{}'".format(choice))
        print(subparser.format_help())

This example should work for python 2.7 and python 3. The example parser is from Python 2.7 documentation on argparse sub-commands.

The only thing left to do is adding a new argument for the complete help, or replacing the built in -h/--help.