Decorator helper method around `add_event_handler`. Example:

        >>> from telethon import TelegramClient, events
        >>> client = TelegramClient(...)
        >>>
        >>> @client.on(events.NewMessage)
        ... async def handler(event):
        ...     ...
        ...
        >>>

        Args:
            event (`_EventBuilder` | `type`):
                The event builder class or instance to be used,
                for instance ``events.NewMessage``.

def on(self, event):
        """
        Decorator helper method around `add_event_handler`. Example:

        >>> from telethon import TelegramClient, events
        >>> client = TelegramClient(...)
        >>>
        >>> @client.on(events.NewMessage)
        ... async def handler(event):
        ...     ...
        ...
        >>>

        Args:
            event (`_EventBuilder` | `type`):
                The event builder class or instance to be used,
                for instance ``events.NewMessage``.
        """
        def decorator(f):
            self.add_event_handler(f, event)
            return f

        return decorator