### Single Container

If you have a simple setup, with a **single container** that then starts multiple **worker processes** (or also just one process), then you could run those previous steps in the same container, right before starting the process with the app. The official Docker image supports this internally.

    * If you depended on that previous behavior, you might need to update your code. As always, make sure your tests pass before merging the upgrade.

## 0.90.1

### Upgrades

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:

* Converting non-JSON request bodies to JSON (e.g. <a href="https://msgpack.org/index.html" class="external-link" target="_blank">`msgpack`</a>).

The analogy in **MongoDB** would be a "collection".

In the code, a `Bucket` represents the main entrypoint of communication with the database.

This utility function will:

* Connect to a **Couchbase** cluster (that might be a single machine).
    * Set defaults for timeouts.
* Authenticate in the cluster.
* Get a `Bucket` instance.
    * Set defaults for timeouts.
* Return it.

```Python hl_lines="12-21"

This could be useful, for example, to expose the same API under different prefixes, e.g. `/api/v1` and `/api/latest`.

This is an advanced usage that you might not really need, but it's there in case you do.

## Include an `APIRouter` in another

The same way you can include an `APIRouter` in a `FastAPI` application, you can include an `APIRouter` in another `APIRouter` using:

    * Generating **automatic errors** returned to the client when the data is invalid.
* **Document** the API using OpenAPI:
    * which is then used by the automatic interactive documentation user interfaces.

This might all sound abstract. Don't worry. You'll see all this in action in the [Tutorial - User Guide](tutorial/index.md){.internal-link target=_blank}.

```Python hl_lines="5-6"
{!../../../docs_src/dependencies/tutorial007.py!}
```

!!! tip
    You can use `async` or regular functions.

    **FastAPI** will do the right thing with each, the same as with normal dependencies.

## A dependency with `yield` and `try`

If you use a `try` block in a dependency with `yield`, you'll receive any exception that was thrown when using the dependency.

  - author: Python Bytes FM
    author_link: https://pythonbytes.fm/
    link: https://pythonbytes.fm/episodes/show/123/time-to-right-the-py-wrongs?time_in_sec=855
    title: FastAPI on PythonBytes
Talks:
  English:
  - author: Jeny Sadadia
    author_link: https://github.com/JenySadadia
    link: https://www.youtube.com/watch?v=uZdTe8_Z6BQ

You can also create an environment variable only for a specific program invocation, that is only available to that program, and only for its duration.

To do that, create it right before the program itself, on the same line:

<div class="termy">

```console
// Create an env var MY_NAME in line for this program call
$ MY_NAME="Wade Wilson" python main.py

    elif field_info is None and depends is None:
        default_value = value if value is not inspect.Signature.empty else Required
        if is_path_param:
            # We might check here that `default_value is Required`, but the fact is that the same
            # parameter might sometimes be a path parameter and sometimes not. See
            # `tests/test_infer_param_optionality.py` for an example.