Protocol incompatibility with Final class variable about mypy HOT 4 OPEN

I think mypy is correct here. Final means that a value is both read-only and immutable. Your protocol definition here does not guarantee immutability of the value of _fields, so Type2, Type3 and Type4 are not compatible with the protocol.

Pyright also generates the same errors as mypy in this case.

A ReadOnly special form was recently added to the type system as part of PEP 705. It currently works only within a TypedDict definition, but there has been some discussion of supporting it in protocols and other classes. If this extension were made to the type system, it would allow you to do what you're trying to do here.

from mypy.

Thank you for the answer. So it seems that it's not currently possible.
However, I find it weird that mypy reports that

Only class variables allowed [...], _fields is an instance variable of "TypeX"

when it is not, it's a class variable. Isn't that a bug?

from mypy.

The protocol defines _fields as a property, which means it should be interpreted as a read-only instance variable. That's how the typing spec indicates properties should be interpreted when they're used in protocols. Mypy is trying to tell you that if you access _fields from the class object Type2, you will get a tuple[str, str] object, but if you try to access _fields from a class object of type Proto2, you will get a property object.

So I understand why you would find the error message somewhat confusing, but it's consistent with the way protocols are defined to work.

from mypy.

I see, thank you.

from mypy.