input: Write a function to parse an XML document and return a list of tag objects. Each tag object should contain the tag's ID and a list of values from its sub-tags. If the XML document is empty, return an empty list.
  
  def parse_xml(text):
      tag_objects = []
  
      root = ET.fromstring(text)
      for child in root:
          tag = Tag(child.attrib["id"])
          for subchild in child:
              tag.add_value(subchild.attrib["value"])
          tag_objects.append(tag)
  
      return tag_objects
  
  class Tag:
      def __init__(self, tag_id):
          self.tag_id = tag_id
          self.values = []
      
      def add_value(self, value):
          self.values.append(value)
  
  assert parse_xml("""<xml>
    <root>
      <child1 id="1">
        <subchild1 value="a">
        </subchild1>
        <subchild2 value="b">
        </subchild2>
      </child1>
      <child2 id="2">
        <subchild3 value="c">
        </subchild3>
        <subchild4 value="d">
        </subchild4>
      </child2>
    </root>
  </xml>""") == [Tag('1', ['a', 'b']), Tag('2', ['c', 'd'])]
  
  assert parse_xml("""<xml></xml>""") == []
output: 
  def parse_xml(text):
      import xml.etree.ElementTree as ET
  
      class Tag:
          def __init__(self, tag_id):
              self.tag_id = tag_id
              self.values = []
          
          def add_value(self, value):
              self.values.append(value)
  
      tag_objects = []
  
      try:
          root = ET.fromstring(text)
          for child in root:
              tag = Tag(child.attrib["id"])
              for subchild in child:
                  tag.add_value(subchild.attrib["value"])
              tag_objects.append(tag)
      except ET.ParseError:
          pass
  
      return tag_objects
  