<img src="/img/tutorial/path-params/image01.png">

!!! check
    Again, just with that same Python type declaration, **FastAPI** gives you automatic, interactive documentation (integrating Swagger UI).

    Notice that the path parameter is declared to be an integer.

| `license_info` | `dict` | The license information for the exposed API. It can contain several...

                "msg": "Value error, name must end in A",
                "input": "modelX",
                "ctx": {"error": HasRepr("ValueError('name must end in A')")},
            }
        )
        | IsDict(
            # TODO remove when deprecating Pydantic v1
            {
                "loc": ("response", "name"),
                "msg": "name must end in A",
                "type": "value_error",
            }

!!! note "Technical Details"
    You could also use `from starlette.testclient import TestClient`.

    **FastAPI** provides the same `starlette.testclient` as `fastapi.testclient` just as a convenience for you, the developer. But it comes directly from Starlette.

!!! tip

The word "**server**" is commonly used to refer to both the remote/cloud computer (the physical or virtual machine) and also the program that is running on that machine (e.g. Uvicorn).

Just keep in mind that when you read "server" in general, it could refer to one of those two things.

## Response with arbitrary `dict`

You can also declare a response using a plain arbitrary `dict`, declaring just the type of the keys and values, without using a Pydantic model.

This is useful if you don't know the valid field/attribute names (that would be needed for a Pydantic model) beforehand.

In this case, you can use `typing.Dict` (or just `dict` in Python 3.9 and above):

=== "Python 3.9+"

    ```Python hl_lines="6"

We can use **OAuth2** to build that with **FastAPI**.

But let's save you the time of reading the full long specification just to find those little pieces of information you need.

Let's use the tools provided by **FastAPI** to handle security.

## How it looks

Let's first just use the code and see how it works, and then we'll come back to understand what's happening.

## Create `main.py`

For example, if you want to read or manipulate the request body before it is processed by your application.

!!! danger
    This is an "advanced" feature.

    If you are just starting with **FastAPI** you might want to skip this section.

## Use cases

Some use cases include:

                            "title": "Price",
                            "exclusiveMinimum": 0.0,
                            "type": "number",
                            "description": "The price must be greater than zero",
                        },
                        "tax": IsDict(
                            {
                                "title": "Tax",

```Python hl_lines="3  6-8"
{!../../../docs_src/sub_applications/tutorial001.py!}
```

### Sub-application

Then, create your sub-application, and its *path operations*.

This sub-application is just another standard FastAPI application, but this is the one that will be "mounted":

```Python hl_lines="11  14-16"
{!../../../docs_src/sub_applications/tutorial001.py!}
```

### Mount the sub-application