Python Negatypes (2021)

`__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 and 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.

Fun followup: turns out that `__subclasshook__` interacts with the new pattern matching. I'm writing a short piece on that now.

> How is this useful?

It seems as though we're talking about a `filter` at the type level.

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.

> How is this useful? No idea.

I love endeavors like this. It’s an interesting feature and a fun read, who cares if it’s useful or not.