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I have a web server written in python which takes an optional argument telling it what IP and port to bind to. Here's the definition:
parser.add_argument(
 '-b', '--bind',
 metavar="[ip]:port",
 type=unicode,
 help="IP and port to bind to."
)
There are several other arguments as well. When the usage is printed (due to parse failures or -h), if the command line is longer than terminal width, it is wrapped. Here is the (relevant) code in argparse.py:311 which deals with this scenario:
# wrap the usage parts if it's too long
text_width = self._width - self._current_indent 
if len(prefix) + len(usage) > text_width: 
 
 # break usage into wrappable parts 
 part_regexp = r'\(.*?\)+|\[.*?\]+|\S+' 
 opt_usage = format(optionals, groups) 
 pos_usage = format(positionals, groups) 
 opt_parts = _re.findall(part_regexp, opt_usage) 
 pos_parts = _re.findall(part_regexp, pos_usage) 
 assert ' '.join(opt_parts) == opt_usage
 assert ' '.join(pos_parts) == pos_usage
Note how part_regexp extracts words, and text surrounded with round/square brackets. Now the argument I defined above, when displayed in usage text, looks like this: [-b [ip]:port]
part_regexp however, will cut it into "[-b [ip]" and ":port]" and concatenated later with " ", it will have an extra space which causes the first assertion to fail.
I fiddled with part_regexp but that proved unreliable at best. I think the opt_parts should be obtained, not from the final formatted text, but from actual argument objects.
For a demonstration, see the attachment.

Yes, this is a known bug (Issue 11874). Patches welcome.

Im kind of surprised this bug has lasted for so many years :)
setting part_regexp to:
r'\(.*?\(.*?\).*?\)+|\[.*?\[.*?\].*?\]+|\S+'
fixes the issue for me, although its not very elegant

New changeset 66f02aa32f1e4adb9f24cf186f8c495399d5ce9b by Nick Coghlan (wim glenn) in branch 'master':
bpo-11874: fix assertion failure in argparse metavar handling (GH-1826)
https://github.com/python/cpython/commit/66f02aa32f1e4adb9f24cf186f8c495399d5ce9b

New changeset 376c272d68cca0975ff0be3d12abf5f67da342d7 by Miss Islington (bot) in branch '3.6':
bpo-11874: fix assertion failure in argparse metavar handling (GH-1826)
https://github.com/python/cpython/commit/376c272d68cca0975ff0be3d12abf5f67da342d7

New changeset 842985f6c70484ed7b8fc30d0bc05aec73236a98 by Miss Islington (bot) in branch '3.7':
bpo-11874: fix assertion failure in argparse metavar handling (GH-1826)
https://github.com/python/cpython/commit/842985f6c70484ed7b8fc30d0bc05aec73236a98

New changeset 4e6bd247aa955056626bf0cf8dc1c65a587b871f by Miss Islington (bot) in branch '2.7':
bpo-11874: fix assertion failure in argparse metavar handling (GH-1826)
https://github.com/python/cpython/commit/4e6bd247aa955056626bf0cf8dc1c65a587b871f