More convenient access to extra asgi scope fields?

[hoodmane]

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Example Code

There is a full gist with example code here, but see description first.
https://gist.github.com/hoodmane/d6f2b6fa3dfb79cafe958edfa3d6aab2

Description

I am trying to get FastAPI working in cloudflare workers + Pyodide. I have a custom asgi server to adapt between our runtime notion of Request/Response and asgi. The request handlers get extra context in an environment parameter which gives access to API keys and such. In this context, the environment parameter is very important so we'd like to make accessing it as simple as possible. From the point of view of an asgi server, the natural place to put it is in the scope. But accessing scope from fastapi isn't all that slick (though it's not terrible either).

We can do:

from fastapi import FastAPI, Request

app = FastAPI()


@app.get("/hello")
async def root(req: Request, x: int = 0):
    env = req.scope["env"]
    return {"message": "Hello World", "SECRET_KEY": env.OPENAI_API_KEY}

but it would be nice to be able to say something like:

from fastapi import FastAPI, Request
from mini_asgi_server import Environment

app = FastAPI()


@app.get("/hello")
async def root(env: Environment, x: int = 0):
    return {"message": "Hello World", "SECRET_KEY": env.OPENAI_API_KEY}

where somehow FastAPI could be told to get Environment type variables from scope["env"]. I looked at the code in dependencies.utils.analyze_param but I didn't find any code paths that look relevant for this. The handling of Request and friends looks quite hard-coded.

1. Is there some way to do this?
2. If not, would you accept a PR to add a way?

I was hoping for something like a pytest fixture where I can create a new one like:

@pytest.fixture
def environment(request: Request) -> Environment:
   return request.scope["env"]

Operating System

Other

Operating System Details

Pyodide

FastAPI Version

0.110.0

Pydantic Version

2.6.3

Python Version

4.12

Additional Context

No response

[n8sty]

I think you can achieve this with a dependency:

def env(request: Request) -> Environment:
    return request.scope["env"]

Then in your route:

@app.get("/hello")
async def root(env: Environment = Depends(env), x: int = 0):
    return {"message": "Hello World", "SECRET_KEY": env.OPENAI_API_KEY}

[hoodmane]

Thanks @n8sty, that's quite helpful.