[Python-ideas] A proper way to bring real interfaces to Python

Hi, all! I believe, almost everybody is familiar with the `abc` package. The problem is that the ABC class is not a separate thing from `object`. So, using ABCs often results in complicated inheritance designs and even in the infamous metaclass conflict. Another problem is that ABC performs checks at the moment an object is being instantiated which isn't exactly the way one expects an interface to work. The obvious way would be to enforce the implementation at the moment `type` for that class is created, i.e. on module execution time. I'm aware there was `PEP 245 -- Python Interface Syntax` which was rejected. However, I agree with the idea that the syntax proposed in this PEP wasn't particularly "pythonish". So, I would like to propose adding a third main object called `interface` in addition to `object` and `type` and to use it to define interface objects. Such interfaces could then be used in the class definition in the following way. ``` class MyInterfaceA(interface): def foo(): pass class MyInterfaceB(interface): def bar(): pass class Implementation(implements=[MyInterfaceA, MyInterfaceB]): def foo(): pass def bar(): pass ``` As a proof of concept, I've implemented a library which implements the idea and allows to use this approach right out of the box — https://pypi.org/project/strict-interfaces/. I would like to get any feedback for the idea and the library. Thanks to all!