How to add a sub-element to xml without changing the rest of the elements?

Question

Python

Here is my code:

import xml.etree.ElementTree as ET
def ban(u_id, reason = None):
  xml_doc = ET.Element('root')
  u_id = str(u_id)
  #id
  user = ET.SubElement(xml_doc, 'user', id = u_id)
  #name
  ET.SubElement(user, 'name').text ="name"
  #reason
  ET.SubElement(user, 'reason').text = reason
  #date
  ET.SubElement(user, 'date').text = 'date_of_ban'
  tt = ET.ElementTree(xml_doc)
  #decoration
  prettify(xml_doc)
  #writing                   
  tt.write('banned.xml', encoding = 'UTF-8', xml_declaration = True)

After executing the function, my xml is overwritten, that is, the remaining subelements are deleted, but the created one remains