'When called in dispatch, do the coerce for C{value} and save the
returned value.'

def dispatch(self, parameterName, value):

    if (value is None):
        raise UsageError(("Parameter    '%s'    requires    an    argument." % (parameterName,)))
    try:
        value = self.coerce(value)
    except ValueError as e:
        raise UsageError(('Parameter    type    enforcement    failed:    %s' % (e,)))
    self.options.opts[parameterName] = value
