## Use Case

Let's start with an example **use case** and then see how to solve it with this.

Let's imagine that you have some **machine learning models** that you want to use to handle requests. 🤖

## Deployment Strategies

There are several ways to do it depending on your specific use case and the tools that you use.

You could **deploy a server** yourself using a combination of tools, you could use a **cloud service** that does part of the work for you, or other possible options.

So, by default, `Header` will convert the parameter names characters from underscore (`_`) to hyphen (`-`) to extract and document the headers.

Also, HTTP headers are case-insensitive, so, you can declare them with standard Python style (also known as "snake_case").

So, you can use `user_agent` as you normally would in Python code, instead of needing to capitalize the first letters as `User_Agent` or something similar.

!!! tip
    If you want something quick to copy and paste, don't use this example, use the last one below.

Then, when you create an instance of that `Settings` class (in this case, in the `settings` object), Pydantic will read the environment variables in a case-insensitive way, so, an upper-case variable `APP_NAME` will still be read for the attribute `app_name`.

Now, there are some cases where you might want to have the **same schema for input and output**.

Probably the main use case for this is if you already have some autogenerated client code/SDKs and you don't want to update all the autogenerated client code/SDKs yet, you probably will want to do it at some point, but maybe not right now.

In that case, you can disable this feature in **FastAPI**, with the parameter `separate_input_output_schemas=False`.

!!! info

You probably want to test the external provider once, but not necessarily call it for every test that runs.

In this case, you can override the dependency that calls that provider, and use a custom dependency that returns a mock user, only for your tests.

### Use the `app.dependency_overrides` attribute

Além disso, os cabeçalhos HTTP não diferenciam maiúsculas de minúsculas, portanto, você pode declará-los com o estilo padrão do Python (também conhecido como "snake_case").

Portanto, você pode usar `user_agent` como faria normalmente no código Python, em vez de precisar colocar as primeiras letras em maiúsculas como `User_Agent` ou algo semelhante.

For example:

```Python
class Cat:
    def __init__(self, name: str):
        self.name = name


fluffy = Cat(name="Mr Fluffy")
```

In this case, `fluffy` is an instance of the class `Cat`.

And to create `fluffy`, you are "calling" `Cat`.

So, a Python class is also a **callable**.

Then, in **FastAPI**, you could use a Python class as a dependency.

        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="4  12  20"
    {!> ../../../docs_src/dependencies/tutorial008.py!}
    ```

And all of them can use `yield`.

In this case `dependency_c`, to execute its exit code, needs the value from `dependency_b` (here named `dep_b`) to still be available.

To create the `SessionLocal` class, use the function `sessionmaker`:

```Python hl_lines="11"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

### Create a `Base` class

Now we will use the function `declarative_base()` that returns a class.

Later we will inherit from this class to create each of the database models or classes (the ORM models):

```Python hl_lines="13"