common_parameters --> read_users
```

This way you write shared code once and **FastAPI** takes care of calling it for your *path operations*.

!!! check
    Notice that you don't have to create a special class and pass it somewhere to **FastAPI** to "register" it or anything similar.

    You just pass it to `Depends` and **FastAPI** knows how to do the rest.

## Share `Annotated` dependencies

    ```Python hl_lines="7"
    {!> ../../../docs_src/query_params_str_validations/tutorial006b.py!}
    ```

!!! info
    If you hadn't seen that `...` before: it is a special single value, it is <a href="https://docs.python.org/3/library/constants.html#Ellipsis" class="external-link" target="_blank">part of Python and is called "Ellipsis"</a>.

Because this dependency function doesn't have any scope requirements itself, we can use `Depends` with `oauth2_scheme`, we don't have to use `Security` when we don't need to specify security scopes.

We also declare a special parameter of type `SecurityScopes`, imported from `fastapi.security`.

This `SecurityScopes` class is similar to `Request` (`Request` was used to get the request object directly).

有一个官方 Docker 镜像，其中包含与 Uvicorn worker一起运行的 Gunicorn，如上一章所述：[服务器工作线程 - Gunicorn 与 Uvicorn](server-workers.md){.internal-link target=_blank}。

该镜像主要在上述情况下有用：[具有多个进程和特殊情况的容器](#containers-with-multiple-processes-and-special-cases)。



* <a href="https://github.com/tiangolo/uvicorn-gunicorn-fastapi-docker" class="external-link" target="_blank">tiangolo/uvicorn-gunicorn-fastapi</a>.


!!! warning

We will create a `PeeweeConnectionState`:

```Python hl_lines="10-19"
{!../../../docs_src/sql_databases_peewee/sql_app/database.py!}
```

This class inherits from a special internal class used by Peewee.

It has all the logic to make Peewee use `contextvars` instead of `threading.local`.

## About "Form Fields"

The way HTML forms (`<form></form>`) sends the data to the server normally uses a "special" encoding for that data, it's different from JSON.

**FastAPI** will make sure to read that data from the right place instead of JSON.

!!! note "Technical Details"

* An optional `client_id` (we don't need it for our example).
* An optional `client_secret` (we don't need it for our example).

!!! info
    The `OAuth2PasswordRequestForm` is not a special class for **FastAPI** as is `OAuth2PasswordBearer`.

    `OAuth2PasswordBearer` makes **FastAPI** know that it is a security scheme. So it is added that way to OpenAPI.

    But remember that when you import `Query`, `Path`, `Header`, and others from `fastapi`, those are actually functions that return special classes.

!!! info
    To declare headers, you need to use `Header`, because otherwise the parameters would be interpreted as query parameters.

## Automatic conversion

* **Deep Learning**: this is a sub-field of Machine Learning, so, the same applies. It's just that there is not a single spreadsheet of numbers to multiply, but a huge set of them, and in many cases, you use a special processor to build and / or use those models.

### Concurrency + Parallelism: Web + Machine Learning

このイメージは、主に上記で説明した状況で役に立つでしょう： [複数のプロセスと特殊なケースを持つコンテナ（Containers with Multiple Processes and Special Cases）](#containers-with-multiple-processes-and-special-cases)

* <a href="https://github.com/tiangolo/uvicorn-gunicorn-fastapi-docker" class="external-link" target="_blank">tiangolo/uvicorn-gunicorn-fastapi</a>.

!!! warning