<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_attachment(self, attachment):
 """Add an attachment to this email :param attachment: Add an attachment to this email :type attachment: Attachment """

        self._attachments = self._ensure_append(attachment, self._attachments)