Introduction to XML Structure

From Flat Text to a Tree

XML data often arrives from a web service, a file, or another system as a flat string of characters. Although that string contains tags, nesting, text, and attributes, Python cannot conveniently search it as a hierarchy until it has been parsed. ElementTree, Python's built-in XML library, provides fromstring for this transformation.

ET.fromstring reads the XML string, interprets its tags, nesting, and attributes, and builds an in-memory tree object. The outermost tag becomes the root element. Elements inside it become child elements, so the parsed result represents the XML document as a nested hierarchy that Python code can query.

Reading Parent and Child Relationships

After parsing, the XML is represented as a hierarchy. The root is the outermost element. An element may contain child elements, text content, and attributes. In a document describing a person, the person element can contain children such as name and email. Elements at the same level are siblings because they share the same parent.

Finding an Element by Tag

The find method searches the tree for the first element matching a supplied tag name. It returns an element object when a match is found. If no matching element exists, it returns None. This means find does not directly return the element's text; it returns the element so that your code can then inspect its text or attributes.

import xml.etree.ElementTree as ET

xml_data = "<person><name>Ada</name><email hide=\"yes\">ada@example.com</email></person>"
root = ET.fromstring(xml_data)
name_element = root.find("name")
email_element = root.find("email")

In this example, fromstring parses the XML string into a tree represented by root. Calling root.find("name") searches that tree for the first name element. Calling root.find("email") searches for the first email element. The variables name_element and email_element refer to the located element objects, not yet to their text values.

Reading Text and Attributes

name_text = name_element.text hide_value = email_element.get("hide") print(name_text) print(hide_value)

Checking Search Results

• Treating find as though it returns text directly.

  find returns an element object or None, not a string.

  Fix: Use name.text after locating the element.

• Ignoring the possibility of None.

  find returns None when no element matches the requested tag.

  Fix: Handle the no-match case before trying to read the result.

• Changing the spelling or capitalization of a tag or attribute.

  XML tag and attribute names are case-sensitive.

  Fix: Match the exact spelling used in the XML.

Practice the Parse-and-Search Process

1. The XML string is the starting text form, not yet a convenient searchable hierarchy. ET.fromstring parses it into a tree with a root and nested child elements. find searches that tree for the first matching tag and returns an element object or None. After locating an element, .text reads its text content and .get("attribute_name") retrieves an attribute value.