Formatter _format_action_invocation(self, action) is the key function.  Notice how it treats positionals and optionals separately.  Also it calls

    default = self._get_default_metavar_for_optional(action)
    args_string = self._format_args(action, default)

to get the string that is placed after each option_string.

'choices' are incorporated via the _metavar_formatter() method (through several function calls).  They do not get their own separate consideration.  Tracing this calling stack is not a trivial task.

But if you want custom behavior at this point in the help, _format_action_invocation() is the function to play with.  You could even add the space for a nonexistent short option at this point.


http://bugs.python.org/issue16468 is an example of a 'choices' thread.  Note that a formatted list of choices is also shown in the 'usage' and error messages.  It's about formatting the choices string, not about where to show it.