的所有子元素（后代）
elem：
all_descendants = list(elem.iter())
一个更完整的示例：
>>> import xml.etree.ElementTree as ET>>> a = ET.Element('a')>>> b = ET.SubElement(a, 'b')>>> c = ET.SubElement(a, 'c')>>> d = ET.SubElement(a, 'd')>>> e = ET.SubElement(b, 'e')>>> f = ET.SubElement(d, 'f')>>> g = ET.SubElement(d, 'g')>>> [elem.tag for elem in a.iter()]['a', 'b', 'e', 'c', 'd', 'f', 'g']排除根本身：
>>> [elem.tag for elem in a.iter() if elem is not a]['b', 'e', 'c', 'd', 'f', 'g']
