$ python -m pip install fastapi uvicorn

from fastapi import FastAPI
api = FastAPI()


@api.get('/')
async def index():
    return {"message": "Hello World"}

from typing import Optional
from fastapi import FastAPI
api = FastAPI()


@api.get("/")
async def read_root():
    return {"Hello": "World"}


@api.get("/items/{item_id}")
async def read_item(item_id: int, q: Optional[str] = None):
    return {"item_id": item_id, "q": q}

from typing import Optional
from fastapi import FastAPI, Header
api = FastAPI()


@api.get("/")
async def info(user_agent: Optional[str] = Header(None)):
    return [{"User-Agent": user_agent}]

from typing import Optional
from fastapi import FastAPI
from pydantic import BaseModel
api = FastAPI()


class Item(BaseModel):
    name: str
    price: float
    is_offer: Optional[bool] = None


@api.get("/")
def read_root():
    return {"Hello": "World"}


@api.get("/items/{item_id}")
def read_item(item_id: int, q: Optional[str] = None):
    return {"item_id": item_id, "q": q}


@api.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}

$ uvicorn main:app --reload
INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO: Started reloader process [28720]
INFO: Started server process [28722]
INFO: Waiting for application startup.
INFO: Application startup complete.