You're supposed to be able to customize runtime (and static) assertions in a few different ways: providing custom assertion, explanation, expected_verb, aggregate_verb, etc. However, this functionality is not working correctly because the assertions' conditions do not catch kwargs. That part is fairly easy to fix (though a little tedious), but this also exposed a minor issue: the way that we are handling the assertion, we are not actually interpolating variables. So you couldn't provide a message like "{left} was not {right}". Seems to me like we should probably support assertion_template, explanation_template, etc. We also need to make expected_verb and aggregate_verb available as parameters, not just class fields (and corresponding _template) versions.