'Given the feature keyword arguments (from `feature_kwargs`) this routine
will construct and return the uniqueness keyword arguments -- a subset
of the feature kwargs.'

def unique_kwargs(self, kwargs):

    if isinstance(self.unique, six.string_types):
        return {self.unique: kwargs[self.unique]}
    else:
        return dict(((fld, kwargs[fld]) for fld in self.unique))
