python lxmlによるxmlフォーマットファイルの読み書き


以前はデータセットフォーマットを変換する際にjsonをxmlファイルに変換する必要があり、lxmlパッケージで操作するのは便利でした.
1.xmlファイルを書く
a)etreeとobjectifyで

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)

   test.xml      ：

```
必要ならanno_treeに他のラベルを付ける場合はappendを使えばいいです.

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)

上の出力は次のようになります.

<annotation>
  <folder>VOC2014_instance/personfolder>
  <filename>test.jpgfilename>
  <source>
    <database>COCOdatabase>
    <annotation>COCOannotation>
    <image>COCOimage>
    <url>http://test.jpgurl>
  source>
  <size>
    <width>800width>
    <height>600height>
    <depth>3depth>
  size>
  <segmented>0segmented>
  <object>
    <name>personname>
    <bndbox>
      <xmin>100xmin>
      <ymin>200ymin>
      <xmax>300xmax>
      <ymax>400ymax>
    bndbox>
    <difficult>0difficult>
  object>
annotation>

b)etreeとSubElementで

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'  #    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.xmlを読む
ここではxpathを用いて必要な要素の値を直接抽出することができる.例えば上のtestを取得したい.xmlファイルのx,y座標:

tree = etree.parse("test.xml")
# get bbox
for bbox in tree.xpath('//bndbox'):   #   bndbox     
    for corner in bbox.getchildren():  #   bndbox       
        print corner.text   # string