Final classes in Python 3.x- something Guido isn't telling me?

Question

This question is built on top of many assumptions. If one assumption is wrong, then the whole thing falls over. I\'m still relatively new to Python and have just entered the

Answer 1

In Python 3.6, you should block subclassing without using a metaclass like this:

class SomeBase:

    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        if cls is not SomeBase:
            raise TypeError("SomeBase does not support polymorphism.  Use composition over inheritance.")


class Derived(SomeBase):
    pass

In Python 3.8, you should also use the final decorator to induce type-checking errors:

from typing import final


@final
class SomeBase:
    ...

Type-checking is done by programs like MyPy, which are optional.