How to use the yagmail.error.YagConnectionClosed function in yagmail

def _prepare_message(self, addresses, subject, contents, attachments, headers):
        """ Prepare a MIME message """
        if self.is_closed:
            raise YagConnectionClosed('Login required again')
        if isinstance(contents, text_type):
            contents = [contents]
        if isinstance(attachments, text_type):
            attachments = [attachments]

        # merge contents and attachments for now.
        if attachments is not None:
            for a in attachments:
                if not os.path.isfile(a):
                    raise TypeError("'{0}' is not a valid filepath".format(a))
            contents = attachments if contents is None else contents + attachments

        has_included_images, content_objects = self._prepare_contents(contents)
        msg = MIMEMultipart()
        if headers is not None:
            # Strangely, msg does not have an update method, so then manually.