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chris.jerdonek wrote:
"Also, to answer a previous question, the three places in which the choices string is used are: in the usage string (separator=','), in the help string when expanding "%(choices)s" (separator=', '), and in the error message text (separator=', ' with repr() instead of str())."
In the usage string, the ',' is used to make a compact representation of the choices. The ', ' separator is used in the help line, where space isn't as tight. 
This 'choices formatting' is called during 'add_argument()' simply as a side effect of checking for valid parameters, especially 'nargs' (that it, is an integer or an acceptable string). Previously 'nargs' errors were not caught until 'parse_args' was used. This is discussed in
http://bugs.python.org/issue9849 Argparse needs better error handling for nargs
http://bugs.python.org/issue16970 argparse: bad nargs value raises misleading message 
On the issue of what error type to raise, my understanding is that 'ArgumentError' is the preferred choice when it affects a particular argument. parse_args() nearly always raises an ArgumentError. Once add_argument has created an action, it too can raise an ArgumentError. ArgumentError provides a standard way of identifying which action is giving the problem.
 
While testing 'metavar="range(0,20)"', I discovered that the usage formatter strips off parenthesis. A regex expression that removes 
excess 'mutually exclusive group' notation is responsible for this. The simple fix is to modify the regex so it distinguishes between ' (...)' and 'range(...)'. I intend to create a new issue for this, since it affects any metavar the includes ().