4.3 The Node class

We'll start by considering the basic Node class. All the other DOM nodes -- Document, Element, Text, and so forth -- are subclasses of Node. It's possible to perform many tasks using just the interface provided by Node.

A note on attributes: In the DOM specification, attributes are defined as being accessible through a pair of accessor methods named get_*() and set_*(). For example, code would read the nodeValue attribute by calling the get_nodeValue() method, and set it by calling set_nodeValue(), passing the new value as the method's argument. This is simple to understand, but it's also a bit clumsy. Therefore, the DOM types all define the __getattr__/__setattr__ magic required to let you simply write node.nodeValue, as if it was simply an ordinary Python attribute.

All DOM nodes have a name, a value, and a type. These are available as the nodeName, nodeValue, and nodeTypeattributes. nodeType and nodeName are read-only; you can't mutate a node into another type of node, nor can you change its name. nodeValue may be changeable, depending on what the node is.

nodeType is used to determine the type of a given node, and is an integer from the following list of constants in the xml.dom.core module:

ATTRIBUTE             CDATA_SECTION            COMMENT               
DOCUMENT              DOCUMENT_FRAGMENT        DOCUMENT_TYPE            
ELEMENT               ENTITY                   ENTITY_REFERENCE      
NOTATION              PROCESSING_INSTRUCTION   TEXT

nodeName and nodeValue are the name and value of the node. For some node types, one of the values may be set to some fixed value. For example, a Text node always has the same value for nodeName: "#text". On the other hand, an Element node has nodeName set to the element's name, such as title or body, and nodeValue is always None.

An instance of Node will usually (but not always) have a parent node. It may also have children, and, if the node's parent has several children, it will also have siblings. The following attributes are available for exploring a node's parent and children:

Attribute	Value
parentNode	Parent of this node
firstChild	First child of this node
lastChild	Last child of this node
previousSibling	Node immediately preceding this node
nextSibling	Node immediately following this node
childNodes	List containing all the children of this node

For each of the attributes in the above table, you can either directly access the attribute, as in node.parentNode, or you can call an accessor method such as node.get_parentNode(). Any of the above attributes can be None if no corresponding node exists; for example, if the node is an only child, both previousSiblingand nextSibling will be None.

Let's look at the values for the first title element in the example tree. That portion of the tree looks like this:

  <Element 'folder'>
   <Element 'title'>
    <Text node 'XML bookmarks'>
   <Element 'bookmark'>
    <Element 'title'>

And the values for the various attributes are:

Attribute	Value
parentNode	folder element
firstChild	Text node 'XML bookmarks'
lastChild	Text node 'XML bookmarks'
previousSibling	codeNone
nextSibling	bookmark element
childNodes	A 1-element list: [ Text node 'XML bookmarks' ]

parentNode can be None if the node is the root of a DOM tree, or if it's a newly created node that hasn't been added to a tree yet.