python - lxml iterparse 与 objectify

Question

如何解析大型 XML 文件并将其元素处理为 ObjectifiedElement(使用 objectify 解析器)。

我没有找到比以下更好的解决方案:

from lxml import etree, objectify
for event, elt in etree.iterparse('onebigfile.xml', tag='MyTag'):
    oelt = objectify.fromstring(etree.tostring(elt))
    my_process(oelt)

如何避免这种中间字符串表示形式？

Answer 1

我觉得真的很好用iterparse构建自定义数据提取器，完全无需使用 objectify。

为了这个示例，我使用了一个看起来有点像这样的 .NET 引用 XML 文件:

<doc>
  <assembly>
    <name>System.IO</name>
  </assembly>
  <members>
    <member name="T:System.IO.BinaryReader">
      <summary>Reads primitive data types as binary values in a specific encoding.</summary>
      <filterpriority>2</filterpriority>
    </member>
    <member name="M:System.IO.BinaryReader.#ctor(System.IO.Stream)">
      <summary>Initializes a new instance of the <see cref="T:System.IO.BinaryReader" /> class based on the specified stream and using UTF-8 encoding.</summary>
      <param name="input">The input stream. </param>
      <exception cref="T:System.ArgumentException">The stream does not support reading, is null, or is already closed. </exception>
    </member>
    <member name="M:System.IO.BinaryReader.#ctor(System.IO.Stream,System.Text.Encoding)">
      <summary>Initializes a new instance of the <see cref="T:System.IO.BinaryReader" /> class based on the specified stream and character encoding.</summary>
      <param name="input">The input stream. </param>
      <param name="encoding">The character encoding to use. </param>
      <exception cref="T:System.ArgumentException">The stream does not support reading, is null, or is already closed. </exception>
      <exception cref="T:System.ArgumentNullException">
        <paramref name="encoding" /> is null. </exception>
    </member>
    <!-- ... many more members like this -->
  </members>
</doc>

假设您想要将所有成员及其名称、摘要和属性提取为像这样的字典列表:

{
  'summary': 'Reads primitive data types as binary values in a specific encoding.', 
  'name': 'T:System.IO.BinaryReader'
}
{
  'summary': 'Initializes a new instance of the ', 
  '@input': 'The input stream. ', 
  'name': 'M:System.IO.BinaryReader.#ctor(System.IO.Stream)'
}
{
  'summary': 'Initializes a new instance of the class based on the specified stream and using UTF-8 encoding.', 
  '@input': 'The input stream. ',
  '@encoding': 'The character encoding to use. ',
  'name': 'M:System.IO.BinaryReader.#ctor(System.IO.Stream,System.Text.Encoding)'
}

你可以这样做:

• 使用lxml.iterparse与 start和 end事件
• 当 <member>元素开始，准备一个新的字典(item)
• 当我们在一个<member>元素，将我们感兴趣的任何内容添加到字典中
• 当 <member>元素结束，完成字典并产生它
• 设置item至 None用作“<member> 的内部/外部”-flag

在代码中:

import lxml
from lxml import etree

def text_content(elt):
    return ' '.join([t.strip() for t in elt.itertext()])

def extract_data(xmlfile):
    item = None

    for event, elt in etree.iterparse(xmlfile, events=['start', 'end']):
        if elt.tag == 'member':
            if event == 'start':
                item = {}
            else:
                item['name'] = elt.attrib['name']
                yield item
                item = None

        if item == None:
            continue

        if event == 'end':
            if elt.tag in ('summary', 'returns'):
                item[elt.tag] = text_content(elt)
                continue

            if elt.tag == 'param':
                item['@' + elt.attrib['name']] = text_content(elt)
                continue


testfile = r'C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETCore\v4.5.1\System.IO.xml'

for item in extract_data(testfile):
    print(item)

通过这种方式，您可以获得最快和最节省内存的解析，并可以很好地控制您查看的数据。使用 objectify会比没有中间体更浪费 tostring()/fromstring() .