First Steps

The simplest FastAPI file could look like this:

from fastapi import FastAPI

app = FastAPI()

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

Copy that to a file main.py.

Run it

Run the live server with:

uvicorn main:app --reload

Check it

Open your browser at http://127.0.0.1:8000.

You will see the JSON response:

{"message": "Hello World"}

Interactive API docs

Now go to http://127.0.0.1:8000/docs.

You will see the automatic interactive API documentation (provided by Swagger UI).