PEP 487 is insane, holy fuck this is awesome

pep487.py

from typing import Any, Dict, List, NamedTuple, Optional, Tuple, TypeVar, Generic, Type
T = TypeVar("T")


class Field(Generic[T]):
    key: str
    t: Type[T]
    readonly: bool

    def __init__(self, t: Type[T], readonly: bool=True) -> None:
        self.t = t
        self.readonly = readonly

    def __get__(self, instance: Any, owner: Any) -> T:
        return instance.__dict__[self.key]

    def __set__(self, instance: Any, new: T):
        assert not self.readonly
        old = instance.__dict__.get(self.key)
        instance.__dict__[self.key] = new
        if old != new:
            self.notify(instance, old, new)

    def notify(self, instance: Any, old: Any, new: Any) -> None:
        print(self, instance, old, new)


class MutableField(Field[T]):
    def __init__(self, t: Type[T]) -> None:
        super().__init__(t, readonly=False)


class BaseModel:
    def __init_subclass__(cls, **kwargs) -> None:
        cls.fields = fields = {}
        for name, attr in cls.__annotations__.items():
            if issubclass(attr.__origin__, Field):
                field_cls = attr.__origin__
                fields[name] = field = field_cls(*attr.__args__)
                setattr(cls, name, field)
                field.key = name

        def __init__(self, *args, **kwargs: Any) -> None:
            for name, value in zip(cls.fields, args):
                self.__dict__[name] = value
            for name in kwargs:
                if name in cls.fields:
                    self.__dict__[name] = kwargs[name]
        cls.__init__ = __init__
        super().__init_subclass__()



# USAGE
import pendulum


class Blob(BaseModel):
    id: Field[int]
    content: Field[bytes]
    last_modified: MutableField[pendulum.Pendulum]


blob = Blob(3, b"hello")
blob.last_modified = pendulum.now()