Basic

• XML：eXtensible Markup Language
• XML is a syntax (serialization format) for data sharing and exchange on the Web
  • Can translate any data to XML
  • Can ship XML over the Web (HTTP)
  • Can input XML into any application

XML Structure

• XML is self-describing
• Schema elements become part of the data(关系型数据记录中schema不算是内容，但是在xml中节点自身也携带内容信息)
• XML is semi-structured
  • missing attributes (could be represented as null in table)
  • repeated attributes (impossible in table)
  • Attributes with different types in different objects
  • Nested structures
  • Heterogeneous contents
• an XML document has a single root element

example:
<bib>
...
  <book price="35">
    <publisher>Addison-Wesley</publisher>
    <author>Serge Abiteboul</author> 
    <author>
      <first-name>Rick</first-name>
      <last-name>Hull</last-name>
    </author> 
    <author age="20">Victor Vianu</author>
    <title>Foundations of Databases</title>
    <year>1995</year>
    <price>38.8</price> 
  </book>
  <book price="55">
    <publisher>Freeman</publisher>
    <author>Jeffrey D. Ullman</author>
    <title>Principles of Database and Knowledge Base Systems</title> 
    <year>1998</year>
  </book> 
... 
</bib>

Querying XML Data

XPath: simple navigation through the tree

lxml

$ pip install lxml

from lxml import etree
f = open('bibs.xml')
tree = etree.parse(f)
print(etree.tostring(tree, pretty_print=True))

for element in tree.xpath("//author"): 
    print(etree.tostring(element))
    print(element.tag, element.text)