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

        self.options.opts[parameterName] = value
When called in dispatch, do the coerce for C{value} and save the
dataset/ETHPy150Open nlloyd/SubliminalCollaborator/libs/twisted/python/usage.py/CoerceParameter.dispatch