[feature request] final classes and member functions

[dgoldstein0]

In some languages (e.g. java) it's possible to declare a class as final so that it can't be subclassed; similarly, it's possible to declare functions as final so that they can't be overridden by sublclasses. It would be great if mypy could support this - so that classes could be annotated as final or their functions as final.

[gvanrossum]

I recall this being proposed as a feature of ABC/ABCMeta as well. I guess the syntax would be pretty obvious: a @final decorator on a method or class.

But the way these things go it would have to be added to PEP 484 first, and that means a discussion in the https://github.com/python/typing/ tracker, and waiting until the release of Python 3.5.3.

We currently don't have a way to define mypy-only features that require new "syntax" like this, although it has occasionally been proposed and isn't technically difficult.

The quickest route to success however would be just to implement your own class decorator specific to your app that somehow sabotages subclassing. It would probably have to add a custom metaclass (or ride piggy-back on one you're already using) so that it can take over control of the class creation machinery. (If you need help with this I can try to construct a prototype and send it to you off-list.)

[gvanrossum]

Closing, since this really needs to be brought up in the typing tracker. Sorry!

[dgoldstein0]

python/typing#286