available, instead of erroring out, the dependency result will be
                `None`.

                This is useful when you want to have optional authentication.

                It is also useful when you want to have authentication that can be
                provided in one of multiple optional ways (for example, in a query
                parameter or in an HTTP Bearer token).
                """

For example an object `orion_cat` (an instance of `Pet`) could have an attribute `orion_cat.type`, for the column `type`. And the value of that attribute could be, e.g. `"cat"`.

These ORMs also have tools to make the connections or relations between tables or entities.

This way, you could also have an attribute `orion_cat.owner` and the owner would contain the data for this pet's owner, taken from the table *owners*.

!!! tip
    A file starting with a dot (`.`) is a hidden file in Unix-like systems, like Linux and macOS.

    But a dotenv file doesn't really have to have that exact filename.

                    "operationId": "read_items_items__get",
                    "parameters": [
                        {
                            "description": "Query string for the items to search in the database that have a good match",
                            "required": False,
                            "deprecated": True,
                            "schema": IsDict(
                                {

## 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. 🤖

The same models are shared among requests, so, it's not one model per request, or one per user or something similar.

We can do better...

## What makes a dependency

Up to now you have seen dependencies declared as functions.

But that's not the only way to declare dependencies (although it would probably be the more common).

The key factor is that a dependency should be a "callable".

A "**callable**" in Python is anything that Python can "call" like a function.

        )
    ]
}
foo_is_short = {
    "detail": [
        IsDict(
            {
                "ctx": {"min_length": 1},
                "loc": ["query", "foo"],
                "msg": "String should have at least 1 character",
                "type": "string_too_short",
                "input": "",
            }
        )
        # TODO: remove when deprecating Pydantic v1
        | IsDict(
            {

```Python hl_lines="2"
@app.get('/')
def results():
    results = some_library()
    return results
```

---

    But `OAuth2PasswordRequestForm` is just a class dependency that you could have written yourself, or you could have declared `Form` parameters directly.

    But as it's a common use case, it is provided by **FastAPI** directly, just to make it easier.

### Use the form data

!!! tip

            if route.status_code is not None:
                status_code = str(route.status_code)
            else:
                # It would probably make more sense for all response classes to have an
                # explicit default status_code, and to extract it from them, instead of
                # doing this inspection tricks, that would probably be in the future