python利用lxml读写xml格式文件

之前在转换数据集格式的时候需要将json转换到xml文件，用lxml包进行操作非常方便。

1. 写xml文件

a) 用etree和objectify

from lxml import etree, objectify

E = objectify.ElementMaker(annotate=False)
anno_tree = E.annotation(
    E.folder('VOC2014_instance'),
    E.filename("test.jpg"),
    E.source(
        E.database('COCO'),
        E.annotation('COCO'),
        E.image('COCO'),
        E.url("http://test.jpg")
    ),
    E.size(
        E.width(800),
        E.height(600),
        E.depth(3)
    ),
    E.segmented(0),
)

etree.ElementTree(anno_tree).write("text.xml", pretty_print=True)

输出的test.xml文件内容如下：

VOC2014_instance/person
test.jpg

COCO
COCO
COCO
http://test.jpg

800
600
3

0

```

如果需要在anno_tree的基础上加其他标签的话用append即可:

E2 = objectify.ElementMaker(annotate=False)
anno_tree2 = E2.object(
    E.name("person"),
    E.bndbox(
        E.xmin(100),
        E.ymin(200),
        E.xmax(300),
        E.ymax(400)
    ),
    E.difficult(0)
)
anno_tree.append(anno_tree2)

上面的输出就变成了:

<annotation>
  <folder>VOC2014_instance/person</folder>
  <filename>test.jpg</filename>
  <source>
    <database>COCO</database>
    <annotation>COCO</annotation>
    <image>COCO</image>
    <url>http://test.jpg</url>
  </source>
  <size>
    <width>800</width>
    <height>600</height>
    <depth>3</depth>
  </size>
  <segmented>0</segmented>
  <object>
    <name>person</name>
    <bndbox>
      <xmin>100</xmin>
      <ymin>200</ymin>
      <xmax>300</xmax>
      <ymax>400</ymax>
    </bndbox>
    <difficult>0</difficult>
  </object>
</annotation>

b) 用etree和SubElement

annotation = etree.Element("annotation")
etree.SubElement(annotation, "folder").text = "VOC2014_instance"
etree.SubElement(annotation, "filename").text = "test.jpg"
source = etree.SubElement(annotation, "source")
etree.SubElement(source, "database").text = "COCO"
etree.SubElement(source, "annotation").text = "COCO"
etree.SubElement(source, "image").text = "COCO"
etree.SubElement(source, "url").text = "http://test.jpg"
size = etree.SubElement(annotation, "size")
etree.SubElement(size, "width").text ='800'  # 必须用string
etree.SubElement(size, "height").text = '600'
etree.SubElement(size, "depth").text = '3'
etree.SubElement(annotation, "segmented").text = '0'
key_object = etree.SubElement(annotation, "object")
etree.SubElement(key_object, "name").text = “person”
bndbox = etree.SubElement(key_object, "bndbox")
etree.SubElement(bndbox, "xmin").text = str(100)
etree.SubElement(bndbox, "ymin").text = str(200)
etree.SubElement(bndbox, "xmax").text = str(300)
etree.SubElement(bndbox, "ymax").text = str(400)
etree.SubElement(key_object, "difficult").text = '0'
doc = etree.ElementTree(annotation)
doc.write(open("test.xml", "w"), pretty_print=True)

2. 读xml

这里可以用xpath直接提取所需的元素的值。比如想要获取上面test.xml文件的x, y坐标：

tree = etree.parse("test.xml")
# get bbox
for bbox in tree.xpath('//bndbox'):   # 获取bndbox元素的内容
    for corner in bbox.getchildren():  # 便利bndbox元素下的子元素
        print corner.text   # string类型

参考

1. http://lxml.de/tutorial.html
2. https://stackoverflow.com/questions/12657043/parse-xml-with-lxml-extract-element-value