###
14.3.4.2 How callbacks are called

All callbacks are called as follows:

    func(option, opt_str, value, parser, *args, **kwargs)

where

`option`
  is the Option instance that's calling the callback
`opt_str`
  is the option string seen on the command-line that's triggering the callback. (If an abbreviated long option was used, `opt_str` will be the full, canonical option string--e.g. if the user puts --foo on the command-line as an abbreviation for --foobar, then `opt_str` will be --foobar.)
`value`
  is the argument to this option seen on the command-line. `optparse` will only expect an argument if `type` is set; the type of `value` will be the type implied by the option's type. If `type` for this option is `None` (no argument expected), then `value` will be `None`. If `nargs` > 1, `value` will be a tuple of values of the appropriate type.
`parser`
  is the OptionParser instance driving the whole thing, mainly useful because you can access some other interesting data through its instance attributes:
  `parser.largs`
    the current list of leftover arguments, ie. arguments that have been consumed but are neither options nor option arguments. Feel free to modify `parser.largs`, e.g. by adding more arguments to it. (This list will become `args`, the second return value of `parse_args()`.)
  `parser.rargs`
    the current list of remaining arguments, ie. with `opt_str` and `value` (if applicable) removed, and only the arguments following them still there. Feel free to modify `parser.rargs`, e.g. by consuming more arguments.
  `parser.values`
    the object where option values are by default stored (an instance of optparse.OptionValues). This lets callbacks use the same mechanism as the rest of `optparse` for storing option values; you don't need to mess around with globals or closures. You can also access or modify the value(s) of any options already encountered on the command-line.
`args`
  is a tuple of arbitrary positional arguments supplied via the `callback_args` option attribute.
`kwargs`
  is a dictionary of arbitrary keyword arguments supplied via `callback_kwargs`.