h4. Working with POMs during development

This client could be a browser with a frontend, a code from someone else, an IoT device, etc.

You could need to tell the client that:

* The client doesn't have enough privileges for that operation.
* The client doesn't have access to that resource.
* The item the client was trying to access doesn't exist.
* etc.

     * @return The deserialized metadata, never {@code null}.
     * @throws IOException If the metadata could not be deserialized.
     * @throws MetadataParseException If the input format could not be parsed.
     */
    Metadata read(File input, Map<String, ?> options) throws IOException, MetadataParseException;

    /**

     * @return The deserialized settings, never {@code null}.
     * @throws IOException If the settings could not be deserialized.
     * @throws SettingsParseException If the input format could not be parsed.
     */
    Settings read(File input, Map<String, ?> options) throws IOException, SettingsParseException;

    /**

     * @return The deserialized toolchains, never {@code null}.
     * @throws IOException If the toolchains could not be deserialized.
     * @throws ToolchainsParseException If the input format could not be parsed.
     */
    PersistedToolchains read(File input, Map<String, ?> options) throws IOException, ToolchainsParseException;

    /**

And then, you could give that JWT token to a user (or bot), and they could use it to perform those actions (drive the car, or edit the blog post) without even needing to have an account, just with the JWT token your API generated for that.

Using these ideas, JWT can be used for way more sophisticated scenarios.

In those cases, several of those entities could have the same ID, let's say `foo` (a user `foo`, a car `foo`, and a blog post `foo`).

///

And then the `admin_email` setting would be set to `"******@****.***"`.

The `app_name` would be `"ChimichangApp"`.

And the `items_per_user` would keep its default value of `50`.

## Settings in another module { #settings-in-another-module }

# OpenAPI Callbacks { #openapi-callbacks }

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

///

An environment variable (also known as "**env var**") is a variable that lives **outside** of the Python code, in the **operating system**, and could be read by your Python code (or by other programs as well).

Environment variables could be useful for handling application **settings**, as part of the **installation** of Python, etc.

## Create and Use Env Vars { #create-and-use-env-vars }

Now, let's see how you could use that to return a custom response.

Let's say that you want to return an <a href="https://en.wikipedia.org/wiki/XML" class="external-link" target="_blank">XML</a> response.