handler not found in main.py {lambda function }

"errorMessage": "Handler 'handler' missing on module 'main'", "errorType": "Runtime.HandlerNotFound",

Code : from fastapi import FastAPI from mangum import Mangum

app = FastAPI()

@app.get("/") def root(): return {"message":"Hello world"}

handler = Mangum(app,)

IN lambda function main.py file . how to resolve?