First App¶

Code¶

The simplest FastAPI file could look like this:

main.py

from fastapi import FastAPI

app = FastAPI()


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

Copy that to a file main.py.

Server¶

Run the live server:

$ uvicorn main:app --reload

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
<span style="color: green;">INFO</span>:     Started reloader process [28720]
<span style="color: green;">INFO</span>:     Started server process [28722]
<span style="color: green;">INFO</span>:     Waiting for application startup.
<span style="color: green;">INFO</span>:     Application startup complete.

Note

The command uvicorn main:app --reload refers to:

main: the file main.py (the Python "module").
app: the object created inside of main.py with the line app = FastAPI().
--reload: make the server restart after code changes. Only use for development.

Tip

In that command, the fragment main:app is equivalent to:

from main import app

In the output, there's a line with something like:

INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

That line shows the URL where your app is being served, in your local machine.

API docs¶

Now you can open the API docs UI at: http://127.0.0.1:8000/docs.