I'm fine with these as a mixin - they are all very generic and unambiguously named. I'd marginally prefer the opt-in mixin over adding them to the base class.

Ideally they'd be matchers, but since I haven't ported that upstream yet, thats asking for more work, and we can always migrate later.

(https://rbtcollins.wordpress.com/2010/05/10/maintainable-pyunit-test-suites/ and http://testtools.readthedocs.io/en/latest/for-test-authors.html#matchers - sorry about the formatting in the blog post, wordpress changed theme details some time after I wrote the post and it now renders horribly :( )