Protocols for types with fields that can be verified

[quodlibetor]

$ pip freeze | rg mypy
mypy-extensions==0.4.1
mypy-mypyc==0.670

I have some complex objects that can be in one of several states once they have been verified, and I'd like to represent that verification as different types, but I'm having a hard time getting it to work with protocols.

I believe that something like this should work:

from __future__ import annotations
from typing import NamedTuple, Protocol, Optional

class Simple:
    two: Optional[int]

    def as_simpl_verified(self) -> SimpleVerified:
        assert self.two is not None
        return self

class SimpleVerified(Protocol):
    two: int

but I get this error:

$ mypy example_simple.py
example_simple.py:11: error: Incompatible return value type (got "Simple", expected "SimpleVerified")
example_simple.py:11: note: Following member(s) of "Simple" have conflicts:
example_simple.py:11: note:     two: expected "int", got "Optional[int]

[quodlibetor]

This is a repost of python/typing#621 because it was made in the wrong place.

[quodlibetor]

Actually I'm reopening this because the solution of adding typing.cast(SimpleVerified, self) seems like it should be unnecessary for a protocol.

[ilevkivskyi]

This is very similar to #5579. This is just another case where binder can potentially apply some restrictions to a "parent expression". This is however even trickier to implement (probably this should be special-cased to only work if there is a protocol context).

[quodlibetor]

Also, actually, worth noting is that I don't think that cast is behaving the way I would like it to, this raises no errors in mypy:

class Simple:
    two: Optional[int]

    def as_simple_verified(self) -> SimpleVerified:
        # assert self.two is not None
        return cast(SimpleVerified, self)


class SimpleVerified(Protocol):
    two: int

but I would really prefer a solution that does result in errors if verification is not complete.