`__subclasshook__` is super useful if you're trying to pass a mock when unit testing code that (despite this not being a best practice) checks its inputs with isinstance <i>and</i> checks for a class that is far more complex to instantiate than would be wise for your test. I can't say I've ever seen it used outside of testing code before, though.
> How is this useful?<p>It seems as though we're talking about a `filter` at the type level.<p>Whatever properties we might like to prove about a random set of types would seem more easily expressed in a positive, rather than negative, mode.