tiangolo.fastapi/docs/tutorial/first-steps.md

The simplest FastAPI file could look like this:

{!tutorial/src/first-steps/tutorial001.py!}

Copy that to a file main.py.

Run the live server:

uvicorn main:app --debug

!!! note The command uvicorn main:app refers to:

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

You will see an output like:

INFO: Started reloader process [17961]
INFO: Started server process [17962]
INFO: Waiting for application startup.
INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

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

Check it

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

You will see the JSON response as:

{"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):

Alternative API docs

And now, go to http://127.0.0.1:8000/redoc.

You will see the alternative automatic documentation (provided by ReDoc):

Recap, step by step

Step 1: import FastAPI

{!tutorial/src/first-steps/tutorial001.py!}

FastAPI is a Python class that provides all the functionality for your API.

Step 2: create a FastAPI "instance"

{!tutorial/src/first-steps/tutorial001.py!}

Here the app variable will be an "instance" of the class FastAPI.

This will be the main point of interaction to create all your API endpoints.

This app is the same one referred by uvicorn in thet command:

uvicorn main:app --debug

If you create your app like:

{!tutorial/src/first-steps/tutorial002.py!}

And put it in a file main.py, then you would call uvicorn like:

uvicorn main:my_awesome_api --debug

Step 3: create an endpoint

{!tutorial/src/first-steps/tutorial001.py!}

The @app.get("/") tells FastAPI that the function right below is an endpoint and that it should go to the path route /.

Step 4: define the endpoint function

{!tutorial/src/first-steps/tutorial001.py!}

This is a Python function.

It will be called by FastAPI whenever it receives a request to the URL "/".

In this case, it is an async function.

---

You could also define it as a normal function instead of async def:

{!tutorial/src/first-steps/tutorial003.py!}

To know the difference, read the section about Concurrency and async / await.

Step 5: return the content

{!tutorial/src/first-steps/tutorial001.py!}

You can return a dict, list, singular values as str, int, etc.

You can also return Pydantic models (you'll see more about that later).

There are many other objects and models that will be automatically converted to JSON.