Allow Final attributes declared in dataclass body without a default value

[sisp]

Feature

It should be allowed to declare a read-only (final) dataclass attribute:

@dataclass
class A:
    x: Final[str]  # error: Final name must be initialized with a value  [misc]

Pitch

I think it's a common scenario where a dataclass attribute should be read-only once the dataclass has been instantiated. It works fine with a default value

@dataclass
class B:
    x: Final[str] = 'abc'  # ok

and with a regular class:

class B:
    x: Final[str]  # ok
    
    def __init__(self, x: str) -> None:
        self.x = x

See the full example on the mypy playground.

[erictraut]

As a simple workaround, you could assign it a field descriptor.

@dataclass
class A:
    x: Final[str] = field()

[sisp]

@erictraut Thanks for suggesting the workaround! A proper fix would still be nice.

[AlexWaygood]

Closing as a duplicate of #5608