The code below, when invoked with -h, prints:

(.env) [andrew@localhost py]$ python -m tests_sa.argparse_bug -h
usage: argparse_bug.py [-h] (-a A | [-b B | -c C)]

options:
-h, --help show this help message and exit
-a A
-b B
-c C

where the final two characters in the usage line are swapped. It should be

usage: argparse_bug.py [-h] (-a A | [-b B | -c C])

or maybe even

usage: argparse_bug.py [-h] (-a A | (-b B | -c C))

from argparse import ArgumentParser

def main():
    parser = ArgumentParser()
    outer = parser.add_mutually_exclusive_group(required=True)
    outer.add_argument('-a')
    inner = outer.add_mutually_exclusive_group()
    inner.add_argument('-b')
    inner.add_argument('-c')
    parser.parse_args()


if __name__ == '__main__':
    main()

There was a bug/issue that addressed problems with nested mutually_exclusive_groups. It should be easy to find.

The problem is that the usage formatter is brittle, building a string and then striping out "unnecessary" characters. I assume the fix handled the default case (not required), but apparently it missed this variation.

I wasn't too involved with that, since in my opinion nesting these groups is useless, and should be avoided. You might as well use one union group. Test the parsing for yourself.

https://bugs.python.org/issue29553
Argparser does not display closing parentheses in nested mutex groups

supposedly fixed the parentheses for nested groups. You can read its discussion and patches to see why it does not handle your case.

I don't see any examples have required groups. [] is used for un-required, () for required. This patch did not change how the usage was generated. It just refined how excess [()] were removed.

Proper handling of groups requires a major rewrite of the usage formatter.

Nesting argument groups and mutually exclusive groups is now deprecated (see bpo-22047). Thank you for the bug report.