python利用lxml读写xml格式的文件

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：

```

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:

VOC2014_instance/person

test.jpg

COCO

COCO

COCO

http://test.jpg

800

600

3

0

person

100

200

300

400

0

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

http://lxml.de/tutorial.html

https://stackoverflow.com/questions/12657043/parse-xml-with-lxml-extract-element-value

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