False negative: override of @final property

[erictraut]

The typing spec indicates that @final should be enforced for properties, but pyright doesn't emit an error in this case.

from typing import final


class Base:
    @final
    @property
    def prop(self) -> int:
        return 1


class Child(Base):
    # This should result in an error.
    @property
    def prop(self) -> int:
        return 2

Refer to this thread.

[Jimmybentley579]

from abc import ABCMeta

class FinalPropertyMeta(ABCMeta):
def new(mcs, name, bases, namespace):
cls = super().new(mcs, name, bases, namespace)
if 'final_properties' not in cls.dict:
cls.final_properties = set()

    # Collect all final properties from current class definition
    cls.__final_properties__.update(
        attr_name 
        for attr_name in namespace 
        if isinstance(getattr(cls.__dict__[attr_name], 'fget', None), property)
           and getattr(namespace[attr_name].fget._is_final_property_, False)
    )
    
    # Check if any base class has defined these properties as final
    for base_class in bases:
        if hasattr(base_class.__dict__.get('__metaclass__', {}), '__bases__'):
            base_final_properties = getattr(base_class.__dict__, '__final_properties__', set())
            intersections = base_final_properties.intersection(cls.__dict__)
            if intersections:
                raise TypeError(f"Cannot override final properties {intersections} in {name}")

    return cls

def _is_final_property(func):
func.is_final_property = True
return func

class FinalProperty(metaclass=FinalPropertyMeta):
pass

class Base(FinalProperty):
@Property @is_final_property
def prop(self) -> int:
return 1

class Child(Base):
# This will raise a TypeError at runtime due to metaclass check.
@Property
def prop(self) -> int:
return 2

[erictraut]

The spec is currently unclear on whether the @final applies to the attribute (which is a descriptor object) or the method (which is wrapped by the descriptor object). This matters because if it applies to the attribute, then it's not possible to define a setter or a deleter on the property. If it applies to the method, then it's not clear what it means for override behaviors if, for example, the getter is marked @final but the setter is not. I'd prefer to have these details ironed out in the spec before I implement this because the implementation will differ significantly between these different interpretations.

[Jimmybentley579]

Thank you. I'm not proceeding at this time on this project.

[erictraut]

This is addressed in pyright 1.1.394.