return isPrimary;
    }

    /**
     * Set primary channel flag
     *
     * @param primary true if primary
     */
    public void setPrimary(boolean primary) {
        this.isPrimary = primary;
    }

    /**
     * Get number of bytes sent
     *
     * @return bytes sent
     */
    public long getBytesSent() {
        return bytesSent.get();
    }

You can also use `File()` with `UploadFile`, for example, to set additional metadata:

{* ../../docs_src/request_files/tutorial001_03_an_py39.py hl[9,15] *}

## Multiple File Uploads { #multiple-file-uploads }

It's possible to upload several files at the same time.

They would be associated to the same "form field" sent using "form data".

To use that, declare a list of `bytes` or `UploadFile`:

    Note over client,operation: Response is already sent, can't change it anymore
    opt Tasks
        operation -->> tasks: Send background tasks
    end
    opt Raise other exception
        tasks -->> tasks: Handle exceptions in the background task code
    end
```

/// info

This includes, for example:

* Email notifications sent after performing an action:
    * As connecting to an email server and sending an email tends to be "slow" (several seconds), you can return the response right away and send the email notification in the background.
* Processing data:

For example, in one of the ways the OAuth2 specification can be used (called "password flow") it is required to send a `username` and `password` as form fields.

The <abbr title="specification">spec</abbr> requires the fields to be exactly named `username` and `password`, and to be sent as form fields, not JSON.

The spec also states that the `username` and `password` must be sent as form data (so, no JSON here).

### `scope` { #scope }

The spec also says that the client can send another form field "`scope`".

The form field name is `scope` (in singular), but it is actually a long string with "scopes" separated by spaces.

If a client tries to send some **extra cookies**, they will receive an **error** response.

Poor cookie banners with all their effort to get your consent for the <abbr title="This is another joke. Don't pay attention to me. Have some coffee for your cookie. ☕">API to reject it</abbr>. 🍪

### The callback path expression { #the-callback-path-expression }

The callback *path* can have an <a href="https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md#key-expression" class="external-link" target="_blank">OpenAPI 3 expression</a> that can contain parts of the original request sent to *your API*.

In this case, it's the `str`:

   * events.
   */
  open fun onOpen(
    eventSource: EventSource,
    response: Response,
  ) {
  }

  /**
   * Invoked when a new event has been sent to the client.
   *
   * @param id The `id` line of the event, might be null.
   * @param type The `event` line of the event, might be null.
   * @param data The `data` line of the event.
   */
  open fun onEvent(

### Update a Hero with `HeroUpdate` { #update-a-hero-with-heroupdate }

We can **update a hero**. For this we use an HTTP `PATCH` operation.

And in the code, we get a `dict` with all the data sent by the client, **only the data sent by the client**, excluding any values that would be there just for being the default values. To do it we use `exclude_unset=True`. This is the main trick. 🪄