我正在使用Python 3.4，正在尝试argparse与子解析器一起使用，并且我希望具有与Python
2.x中类似的行为，如果我不提供位置参数（以指示子解析器/子程序）我会收到一条有用的错误消息。即，随着python2我将得到以下错误消息：

$ python2 subparser_test.py    
usage: subparser_test.py [-h] {foo} ...
subparser_test.py: error: too few arguments

我正在required按照http://codingdict.com/questions/172394建议设置属性，但是这给了我Python
3.4.0错误： TypeError: sequence item 0: expected str instance, NoneType found-完全追溯：

$ python3 subparser_test.py    
Traceback (most recent call last):
  File "subparser_test.py", line 17, in <module>
    args = parser.parse_args()
  File "/usr/local/Cellar/python3/3.4.0/Frameworks/Python.framework/Versions/3.4/lib/python3.4/argparse.py", line 1717, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/local/Cellar/python3/3.4.0/Frameworks/Python.framework/Versions/3.4/lib/python3.4/argparse.py", line 1749, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/local/Cellar/python3/3.4.0/Frameworks/Python.framework/Versions/3.4/lib/python3.4/argparse.py", line 1984, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, NoneType found

这是我的程序subparser_test.py-改编自https://docs.python.org/3.2/library/argparse.html#sub-
commands：

import argparse

# sub-command functions
def foo(args):
    print('"foo()" called')

# create the top-level parser
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers()
subparsers.required = True

# create the parser for the "foo" command
parser_foo = subparsers.add_parser('foo')
parser_foo.set_defaults(func=foo)

args = parser.parse_args()
args.func(args)