Cannot override class variable -- using `Final` and `ClassVar`

[christianbundy]

Bug Report

PEP 591 indicates:

Type checkers should infer a final attribute that is initialized in a class body as being a class variable. Variables should not be annotated with both ClassVar and Final.

But in practice, Mypy displays a type error when you use Final on a subclass where the parent class used a ClassVar.

To Reproduce

from typing import ClassVar, Final, ClassVar, Literal


class Animal:
    description: ClassVar[str] = "hard to generalize"
    
    
class Dog(Animal):
    description: Final[str] = "very good"

https://mypy-play.net/?mypy=latest&python=3.11&gist=69298f23e25b94da7dadc694769db16d

Expected Behavior

No error.

Actual Behavior

Cannot override class variable (previously declared on base class "Animal") with instance variable  [misc]

You may be tempted to work around this with:

class Dog(Animal):
    description: Final[ClassVar[str]] = "very good"

But then Mypy outputs:

Variable should not be annotated with both ClassVar and Final  [misc]

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11

[AlexWaygood]

I think mypy's behaviour is correct here. Your base class has an attribute description that's settable on the class object itself. But your subclass has an attribute description that's read-only in all situations. That seems like an incompatible override that violates the Liskov Substitution Principle.

Remember that Final does not just mean "the name cannot be overridden in subclasses"; it also means "the name cannot be reassigned to in the same scope".

[christianbundy]

Makes sense to me, thanks a lot for the explanation!