vercel dev with Python runtime downloads index.py file instead of running function

[isaacharrisholt]

Page to Investigate

https://vercel.com/docs/concepts/functions/serverless-functions/runtimes/python

Steps to Reproduce

I have a monorepo containing a number of Vercel projects. One of said Vercel projects is for functions only. I'm trying to create a Python function within the /api/test directory, like so:

repo
└── functions (Vercel project root)
    └── api
        └── test
            └── index.py

The Python function is using FastAPI:

from fastapi import FastAPI

app = FastAPI()


@app.get("/api/test")
def hello_world():
    return "Hello, world!"

However, I've had this issue with the HTTP example provided in the docs.

Essentially, when I run vercel dev from the root of my repo (which has been vercel linked to my functions project) and navigate to localhost:3000/api/test, my browser tries to download the index.py file instead of running the function.

If I rename index.py to foo.py and head to /api/test/foo, I get a 502 error:

using Asynchronous Server Gateway Interface (ASGI)
<class 'TypeError'>
TypeError: Object prototype may only be an Object or null: undefined
    at Function.setPrototypeOf (<anonymous>)
    at new LambdaError (/home/isaac/.nvm/versions/node/v18.15.0/lib/node_modules/vercel/dist/index.js:8710:16)
    at Lambda.<anonymous> (/home/isaac/.nvm/versions/node/v18.15.0/lib/node_modules/vercel/dist/index.js:8843:27)
    at Generator.next (<anonymous>)
    at fulfilled (/home/isaac/.nvm/versions/node/v18.15.0/lib/node_modules/vercel/dist/index.js:8757:58)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)

None of this happens when deployed though - it all works just fine.

Any suggestions as to what I'm doing wrong?

[hassanpasha5630]

I was having the same issue so I did something like this "fastapi-dev": "pip3 install -r requirements.txt && python3 -m uvicorn api.index:app --reload",

I would just edit the run command for example if I updated index to foo I would do api.foo:app this worked for me

Hope this helps

[SaraswatAnushka]

i am not using fastapi but I am having the same issue
Can anybody help, I don't know how to work with flask and this deployment

[HammadAwan424]

If you set Vercel project root to default that is to the root of repo (just don't modify vercel root path) then you can go with:
{
"builds": [
{
"src": "/functions/api/index.py",
"use": "@vercel/python"
}
],
"rewrites": [
{
"source": "/(.*)",
"destination": "/functions/api/index.py"
}
]
}