I have a simple FastAPI application. It has a lifespan function that defines a custom attribute on the app object (an HTTPX AsyncClient), and a route function that tries to access said attribute.

from fastapi import FastAPI, Request
from contextlib import asynccontextmanager
import uvicorn
import httpx

@asynccontextmanager
async def lifespan(_app: FastAPI):
    _app.client = httpx.AsyncClient(base_url="https://example.com")
    yield
    await _app.client.aclose()

app = FastAPI(lifespan=lifespan)

@app.get("/test")
async def test(request: Request):
    return str(request.app.client)

if __name__ == "__main__":
    uvicorn.run(app)

When running the app locally, the route function is able to access the attribute without a problem.
When the app is deployed to Vercel however, I get the following error.

AttributeError: 'FastAPI' object has no attribute 'client'

How come the attribute is accessible locally, but not on Vercel?

Project info:

• Local Python version: 3.12.4 w/ virtual environment

• Vercel Python version: 3.12 according to Vercel’s docs

• requirements.txt

  annotated-types==0.7.0
  anyio==4.4.0
  certifi==2024.7.4
  click==8.1.7
  colorama==0.4.6
  fastapi==0.112.0
  h11==0.14.0
  httpcore==1.0.5
  httpx==0.27.0
  idna==3.7
  pydantic==2.8.2
  pydantic_core==2.20.1
  sniffio==1.3.1
  starlette==0.37.2
  typing_extensions==4.12.2
  uvicorn==0.30.5
  

• vercel.json

  {
    "version": 2,
    "builds": [{ "src": "main.py", "use": "@vercel/python" }],
    "routes": [{ "src": "/(.*)", "dest": "main.py" }]
  }
  

I’ve tried:

• Modifying FastAPI’s own lifespan example to suit my needs, but the variable would always be a NoneType.

  client = None
  
  @asynccontextmanager
  async def lifespan(_app: FastAPI):
      client = httpx.AsyncClient(base_url="example.com")
      yield
      await client.aclose()
  
  app = FastAPI(lifespan=lifespan)
  

  AttributeError: 'NoneType' object has no attribute 'get'
  

• Letting FastAPI install its own version of Starlette by removing it from requirements.txt, as suggested by this issue.