You can customize the JSON response from FastAPI using the @app.get decorator. For example:

from typing import Optional
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/{item_id}")
def read_item(item_id: int):
    if item_id == 1:
        return {item_id: "One"}
    else:
        return {item_id: None}

Acessing 127.0.0.1:8000/items/1 returns:

{"1":"One"}

And 127.0.0.1:8000/items/2 returns:

{"2":"null}