input: Write a function to parse an XML document and return a list of tag objects. Each tag object should contain an ID and a list of values corresponding to its subchild elements. If the provided XML document is empty, the function should 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 [tag.tag_id for tag in 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>')] == ['1', '2']
  assert [value for tag in 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>') for value in tag.values] == ['a', 'b', 'c', 'd']
  assert parse_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 = []
  
      if not text:
          return 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
  