[Python] Type hinting a python descriptor implementing read-only instance attributes after...

I have a couple of data structure classes that are intended to have public read-only attributes. I cannot use dataclasses or namedtuples as I need the ability to define args, kwargs in the child classes inheriting these classes.

One way I can set this up is simply having read-only properties but at some point, my data structure can have more than 20 attributes and writing a property for each one of them feels cumbersome.

I think the best solution for me would be to implement a descriptor and I have an implementation which works but mypy seems to be struggling with it:

from typing import Type, TypeVar, Generic

_Object = TypeVar("_Object")
_Value = TypeVar("_Value")


class ReadOnlyAttribute(Generic[_Value]):
def __set_name__(self, owner: Type[_Object], name: str) -> None:
self.public_name = name
self.private_name = f"_{name}"

def __get__(self, instance: _Object, owner: Type[_Object]) -> _Value:
return getattr(instance, self.private_name)

def __set__(self, instance: _Object, value: _Value) -> None:
if hasattr(instance, self.private_name):
raise AttributeError(
f"Cannot set read-only attribute '{self.public_name}' of '{instance}'"
)

setattr(instance, self.private_name, value)


class Test:
# error: Incompatible types in assignment (expression has type "ReadOnlyAttribute[Never]", variable has type "str") [assignment]
foo: str = ReadOnlyAttribute()

def __init__(self):
self.foo = "foooooo"

# Revealed type is "builtins.str"
reveal_type(Test().foo)


It is able to infer the type correctly but is giving an error about incompatible types in assignment. I think it's because I am type-hinting self.foo as a str but assigning an instance of ReadOnlyAttribute?

However, without doing this and using Generic, reveal_type does not perform as expected and returns Never.

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