# Tutorial - User Guide

This tutorial shows you how to use **FastAPI** with most of its features, step by step.

Each section gradually builds on the previous ones, but it's structured to separate topics, so that you can go directly to any specific one to solve your specific API needs.

It is also built to work as a future reference.

So you can come back and see exactly what you need.

## Run the code

    {!> ../../../docs_src/body/tutorial001_py310.py!}
    ```

=== "Python 3.8+"

    ```Python hl_lines="18"
    {!> ../../../docs_src/body/tutorial001.py!}
    ```

...and declare its type as the model you created, `Item`.

## Results

With just that Python type declaration, **FastAPI** will:

* Read the body of the request as JSON.
* Convert the corresponding types (if needed).

  - author: Uber Engineering
    author_link: https://eng.uber.com
    link: https://eng.uber.com/ludwig-v0-2/
    title: 'Uber: Ludwig v0.2 Adds New Features and Other Improvements to its Deep Learning Toolbox [including a FastAPI server]'
  - author: Maarten Grootendorst
    author_link: https://www.linkedin.com/in/mgrootendorst/

      "type": "type_error.integer"
    }
  ],
  "body": {
    "title": "towel",
    "size": "XL"
  }
}
```

#### FastAPI's `HTTPException` vs Starlette's `HTTPException`

**FastAPI** has its own `HTTPException`.

And **FastAPI**'s `HTTPException` error class inherits from Starlette's `HTTPException` error class.

* Then it starts `Listening at: http://0.0.0.0:80`.
* Then it detects that it has to use the worker class at `uvicorn.workers.UvicornWorker`.
* And then it starts **4 workers**, each with its own PID: `19511`, `19513`, `19514`, and `19515`.

    The `Union` in `Union[str, None]` will allow your editor to give you better support and detect errors.

## Additional validation

We are going to enforce that even though `q` is optional, whenever it is provided, **its length doesn't exceed 50 characters**.

### Import `Query` and `Annotated`

To achieve that, first import:

* `Query` from `fastapi`
* `Annotated` from `typing` (or from `typing_extensions` in Python below 3.9)

Before OpenAPI 3.1.0, OpenAPI used an older and modified version of **JSON Schema**.

JSON Schema didn't have `examples`, so OpenAPI added it's own `example` field to its own modified version.

OpenAPI also added `example` and `examples` fields to other parts of the specification:

                exclude_none=exclude_none,
            )

        def __hash__(self) -> int:
            # Each ModelField is unique for our purposes, to allow making a dict from
            # ModelField to its JSON Schema.
            return id(self)

    def get_annotation_from_field_info(
        annotation: Any, field_info: FieldInfo, field_name: str
    ) -> Any:
        return annotation

                the URL. [CommonMark syntax](https://commonmark.org/) MAY be used for
                rich text representation.
                * `variables`: A `dict` between a variable name and its value. The value
                    is used for substitution in the server's URL template.

                Read more in the

async def run_endpoint_function(
    *, dependant: Dependant, values: Dict[str, Any], is_coroutine: bool
) -> Any:
    # Only called by get_request_handler. Has been split into its own function to
    # facilitate profiling endpoints, since inner functions are harder to profile.
    assert dependant.call is not None, "dependant.call must be a function"

    if is_coroutine: