The simplest FastAPI file could look like this:

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

Copy that to a file `main.py`.

Run the live server:

```bash
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:

```hl_lines="4"
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 <a href="http://127.0.0.1:8000" target="_blank">http://127.0.0.1:8000</a>. 

You will see the JSON response as:

```JSON
{"hello": "world"}
```

### Interactive API docs

Now go to <a href="http://127.0.0.1:8000/docs" target="_blank">http://127.0.0.1:8000/docs</a>. 

You will see the automatic interactive API documentation (provided by <a href="https://github.com/swagger-api/swagger-ui" target="_blank">Swagger UI</a>):

![Swagger UI](https://fastapi.tiangolo.com/img/index/index-01-swagger-ui-simple.png)


### Alternative API docs

And now, go to <a href="http://127.0.0.1:8000/redoc" target="_blank">http://127.0.0.1:8000/redoc</a>.

You will see the alternative automatic documentation (provided by <a href="https://github.com/Rebilly/ReDoc" target="_blank">ReDoc</a>):

![ReDoc](https://fastapi.tiangolo.com/img/index/index-02-redoc-simple.png)

## Recap, step by step

### Step 1: import `FastAPI`

```Python hl_lines="1"
{!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"

```Python hl_lines="3"
{!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:

```bash
uvicorn main:app --debug
```

If you create your app like:

```Python hl_lines="3"
{!tutorial/src/first-steps/tutorial002.py!}
```

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

```bash
uvicorn main:my_awesome_api --debug
```

### Step 3: create an endpoint

```Python hl_lines="6"
{!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

```Python hl_lines="7"
{!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`:

```Python hl_lines="7"
{!tutorial/src/first-steps/tutorial003.py!}
```

To know the difference, read the section about [Concurrency and `async` / `await`](/async/).

### Step 5: return the content

```Python hl_lines="8"
{!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.