The bug is orthogonal, you can trigger it without the `required=` keyword argument via the (currently suggested) monkeypatch workaround which restores the pre-3.3 behaviour:

import argparse

parser = argparse.ArgumentParser()
subp = parser.add_subparsers()
subp.add_parser('test')
subp.required = True
parser.parse_args()


$ python3 test.py
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    parser.parse_args()
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/argparse.py", line 1730, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/argparse.py", line 1762, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/argparse.py", line 1997, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, NoneType found


Also note that when `dest` is specified it works fine:


import argparse

parser = argparse.ArgumentParser()
subp = parser.add_subparsers(dest='cmd')
subp.add_parser('test')
subp.required = True
parser.parse_args()

$ python3 test.py
usage: test.py [-h] {test} ...
test.py: error: the following arguments are required: cmd