How to convert TypedDict to a dataclass_transform behavior?

[Kludex]

Hi there 👋

I'm one of the maintainers of Starlette, and I need help.

In Starlette (ASGI), we have Lifespan events (startup and shutdown). There, you can store data that will be used on your endpoints (see line yield {"potato": "potato"} below for how to store).

And... There are two ways to retrieve this potato in the endpoint:

1. request.scope["state"]["potato"], since state is a dictionary.
2. request.state.potato, since Request.state actually wraps the above dictionary, see Request/State for more details.

The first is an ASGI concept, and the second is specific to Starlette. Deprecating the second is not an option, but we can tweak the implementation as much as wanted if it doesn't break our users' applications.

Anyway... Is there a way to type hint the Request/State in a way the following reveal_types shows the type of potato?

from contextlib import asynccontextmanager
from typing import AsyncGenerator, TypedDict

from starlette.applications import Starlette
from starlette.requests import Request
from starlette.responses import Response
from starlette.routing import Route


class LifespanState(TypedDict):
    potato: str


@asynccontextmanager
async def lifespan(app: Starlette) -> AsyncGenerator[LifespanState, None]:
    yield {"potato": "potato"}


async def homepage(request: Request) -> Response:
    # I want `request.state.potato` to be `str`.
    reveal_type(request.state.potato)

    # I also want `request.scope["state"]["potato"]` to be `str`.
    reveal_type(request.scope["state"]["potato"])

    return Response(content="Hello, world!")


app = Starlette(routes=[Route("/", endpoint=homepage)])

Note

Title may be misleading - change it if you want. 👀

[developoohs]

[Kludex]

No. It doesn't solve the issue.

[sobolevn]

You can make request.scope["state"] generic, but you cannot convert TypedDict to a dataclass.

Here's how .scope['state'] would work:

_ScopeStateT = TypeVar('_ScopeStateT')

class Scope(TypedDict, Generic[_ScopeStateT]):
     state: _ScopeStateT

class Request(Generic[_ScopeStateT]):
     scope: Scope[_ScopeStateT]

And then:

async def homepage(request: Request[LifespanState]) -> Response:
    reveal_type(request.scope["state"]["potato"])  # N: Revealed type is "builtins.str"

[Kludex]

Yeah, that's fine for the first case. I should have been more explicit that I'm looking to satisfy the second case.

[...] but you cannot convert TypedDict to a dataclass.

Yeah... I know, but I don't know why. Would you mind sharing the reason? Also, I'm more interested in finding the less disruptive solution here that will make request.state type safe.

(Thanks @sobolevn 🙏)

[Daverball]

This just wasn't part of PEP-681's scope. So it's not possible right now without adding a second TypeVar for the other way to access the data. So the best you can do right now is make request.state an Any or a Protocol with a __getattr__ which returns Any.

The following proposal might make something along the lines of what you want possible: #1412

[sobolevn]

You motivated me to add this support to https://github.com/wemake-services/mypy-extras/
You can watch it for the next releases.

[Kludex]

This just wasn't part of PEP-681's scope. So it's not possible right now without adding a second TypeVar for the other way to access the data. So the best you can do right now is make request.state an Any or a Protocol with a __getattr__ which returns Any.

This doesn't look like an acceptable API.

The following proposal might make something along the lines of what you want possible: #1412

This does look like something that will solve my issue! 🤩 Thanks!

[Kludex]

You motivated me to add this support to wemake-services/mypy-extras You can watch it for the next releases.

Watching! Thanks! ❤️

[Kludex]

I really want to improve the experience of FastAPI/Starlette users, and having something like this would be great.