如何在argparse的帮助消息中包含所有子命令的帮助消息？

import argparse

# 创建解析器
parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)

# 创建子命令解析器
subparsers = parser.add_subparsers(title='subcommands', dest='subcommand', metavar='<subcommand>')

# 子命令1
subparser1 = subparsers.add_parser('command1', help='This is command 1 help')
subparser1.add_argument('arg1', help='Argument 1')

# 子命令2
subparser2 = subparsers.add_parser('command2', help='This is command 2 help')
subparser2.add_argument('arg2', help='Argument 2')

# 获取所有子命令的帮助消息
subcommands_help = ''
for name, subparser in subparsers.choices.items():
    subcommands_help += f'\n{name}: {subparser.description}'

# 设置帮助消息
parser._positionals.title = 'Positional arguments'
parser._optionals.title = 'Optional arguments'
parser._subparsers.title = 'Subcommands'
parser._subparsers.help = subcommands_help

# 解析命令行参数
args = parser.parse_args()

# 打印帮助消息
parser.print_help()

usage: program.py [-h] <subcommand> ...

positional arguments:
  <subcommand>
    command1   This is command 1 help
    command2   This is command 2 help

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