argparse suffers from a failing assertion when formatting a long usage message with an option whose metavar contains a comma. This can be seen in the attached testcase, which fails with,

Traceback (most recent call last):
  File "/home/ben/hi.py", line 24, in <module>
    args = parser.parse_args()
  File "/usr/lib/python2.7/argparse.py", line 1688, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib/python2.7/argparse.py", line 1720, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python2.7/argparse.py", line 1937, in _parse_known_args
    self.error(_('too few arguments'))
  File "/usr/lib/python2.7/argparse.py", line 2360, in error
    self.print_usage(_sys.stderr)
  File "/usr/lib/python2.7/argparse.py", line 2322, in print_usage
    self._print_message(self.format_usage(), file)
  File "/usr/lib/python2.7/argparse.py", line 2278, in format_usage
    return formatter.format_help()
  File "/usr/lib/python2.7/argparse.py", line 279, in format_help
    help = self._root_section.format_help()
  File "/usr/lib/python2.7/argparse.py", line 209, in format_help
    func(*args)
  File "/usr/lib/python2.7/argparse.py", line 330, in _format_usage
    assert ' '.join(opt_parts) == opt_usage
AssertionError

This failure is apparently due to a regular expression used to wrap  the usage message. The solution to this is unclear, but it seems to me using a regular expression here is a hack at best.

My rewrite of _format_actions_usage in http://bugs.python.org/issue11874 should take care of this issue.  It keeps the groups and actions separate until the final formatting step, bypassing the regular expression parsing.

Thanks @paul.j3 for the PR. The PR in the linked issue was merged and I cannot reproduce the original assertion error in the latest 2.7 branch with the script.

$ ./python.exe
Python 2.7.15+ (remotes/upstream/2.7:10be1d3f80, Sep 22 2018, 22:10:28)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
$ ./python.exe ../backups/bpo16360.py
usage: bpo16360.py [-h] [-w TIME] [-r N] [-j TIME] [-W TIME]
                   [-y COUNT|max|avg] [-o FILE] [-c CHANNEL[=LABEL],...]
                   FILE [FILE ...]
bpo16360.py: error: too few arguments


Thanks

I'm closing this since the https://bugs.python.org/issue11874 fix also handles this issue.