xml.dom — The Document Object Model API
New in version 2.0.
The Document Object Model, or “DOM,” is a cross-language API from the World Wide
Web Consortium (W3C) for accessing and modifying XML documents. A DOM
implementation presents an XML document as a tree structure, or allows client
code to build such a structure from scratch. It then gives access to the
structure through a set of objects which provided well-known interfaces.
The DOM is extremely useful for random-access applications. SAX only allows you
a view of one bit of the document at a time. If you are looking at one SAX
element, you have no access to another. If you are looking at a text node, you
have no access to a containing element. When you write a SAX application, you
need to keep track of your program’s position in the document somewhere in your
own code. SAX does not do it for you. Also, if you need to look ahead in the
XML document, you are just out of luck.
Some applications are simply impossible in an event driven model with no access
to a tree. Of course you could build some sort of tree yourself in SAX events,
but the DOM allows you to avoid writing that code. The DOM is a standard tree
representation for XML data.
The Document Object Model is being defined by the W3C in stages, or “levels” in
their terminology. The Python mapping of the API is substantially based on the
DOM Level 2 recommendation.
DOM applications typically start by parsing some XML into a DOM. How this is
accomplished is not covered at all by DOM Level 1, and Level 2 provides only
limited improvements: There is a DOMImplementation object class which
provides access to Document creation methods, but no way to access an
XML reader/parser/Document builder in an implementation-independent way. There
is also no well-defined way to access these methods without an existing
Document object. In Python, each DOM implementation will provide a
function getDOMImplementation(). DOM Level 3 adds a Load/Store
specification, which defines an interface to the reader, but this is not yet
available in the Python standard library.
Once you have a DOM document object, you can access the parts of your XML
document through its properties and methods. These properties are defined in
the DOM specification; this portion of the reference manual describes the
interpretation of the specification in Python.
The specification provided by the W3C defines the DOM API for Java, ECMAScript,
and OMG IDL. The Python mapping defined here is based in large part on the IDL
version of the specification, but strict compliance is not required (though
implementations are free to support the strict mapping from IDL). See section
Conformance for a detailed discussion of mapping requirements.
Module Contents
The xml.dom contains the following functions:
-
xml.dom.registerDOMImplementation(name, factory)
- Register the factory function with the name name. The factory function
should return an object which implements the DOMImplementation
interface. The factory function can return the same object every time, or a new
one for each call, as appropriate for the specific implementation (e.g. if that
implementation supports some customization).
-
xml.dom.getDOMImplementation([name[, features]])
Return a suitable DOM implementation. The name is either well-known, the
module name of a DOM implementation, or None. If it is not None, imports
the corresponding module and returns a DOMImplementation object if the
import succeeds. If no name is given, and if the environment variable
PYTHON_DOM is set, this variable is used to find the implementation.
If name is not given, this examines the available implementations to find one
with the required feature set. If no implementation can be found, raise an
ImportError. The features list must be a sequence of (feature,
version) pairs which are passed to the hasFeature() method on available
DOMImplementation objects.
Some convenience constants are also provided:
-
xml.dom.EMPTY_NAMESPACE
The value used to indicate that no namespace is associated with a node in the
DOM. This is typically found as the namespaceURI of a node, or used as
the namespaceURI parameter to a namespaces-specific method.
New in version 2.2.
-
xml.dom.XML_NAMESPACE
The namespace URI associated with the reserved prefix xml, as defined by
Namespaces in XML (section 4).
New in version 2.2.
-
xml.dom.XMLNS_NAMESPACE
The namespace URI for namespace declarations, as defined by Document Object
Model (DOM) Level 2 Core Specification (section 1.1.8).
New in version 2.2.
-
xml.dom.XHTML_NAMESPACE
The URI of the XHTML namespace as defined by XHTML 1.0: The Extensible
HyperText Markup Language (section 3.1.1).
New in version 2.2.
In addition, xml.dom contains a base Node class and the DOM
exception classes. The Node class provided by this module does not
implement any of the methods or attributes defined by the DOM specification;
concrete DOM implementations must provide those. The Node class
provided as part of this module does provide the constants used for the
nodeType attribute on concrete Node objects; they are located
within the class rather than at the module level to conform with the DOM
specifications.
Objects in the DOM
The definitive documentation for the DOM is the DOM specification from the W3C.
Note that DOM attributes may also be manipulated as nodes instead of as simple
strings. It is fairly rare that you must do this, however, so this usage is not
yet documented.
An additional section describes the exceptions defined for working with the DOM
in Python.
DOMImplementation Objects
The DOMImplementation interface provides a way for applications to
determine the availability of particular features in the DOM they are using.
DOM Level 2 added the ability to create new Document and
DocumentType objects using the DOMImplementation as well.
-
DOMImplementation.hasFeature(feature, version)
- Return true if the feature identified by the pair of strings feature and
version is implemented.
-
DOMImplementation.createDocument(namespaceUri, qualifiedName, doctype)
- Return a new Document object (the root of the DOM), with a child
Element object having the given namespaceUri and qualifiedName. The
doctype must be a DocumentType object created by
createDocumentType(), or None. In the Python DOM API, the first two
arguments can also be None in order to indicate that no Element
child is to be created.
-
DOMImplementation.createDocumentType(qualifiedName, publicId, systemId)
- Return a new DocumentType object that encapsulates the given
qualifiedName, publicId, and systemId strings, representing the
information contained in an XML document type declaration.
Node Objects
All of the components of an XML document are subclasses of Node.
-
Node.nodeType
- An integer representing the node type. Symbolic constants for the types are on
the Node object: ELEMENT_NODE, ATTRIBUTE_NODE,
TEXT_NODE, CDATA_SECTION_NODE, ENTITY_NODE,
PROCESSING_INSTRUCTION_NODE, COMMENT_NODE,
DOCUMENT_NODE, DOCUMENT_TYPE_NODE, NOTATION_NODE.
This is a read-only attribute.
-
Node.parentNode
- The parent of the current node, or None for the document node. The value is
always a Node object or None. For Element nodes, this
will be the parent element, except for the root element, in which case it will
be the Document object. For Attr nodes, this is always
None. This is a read-only attribute.
-
Node.attributes
- A NamedNodeMap of attribute objects. Only elements have actual values
for this; others provide None for this attribute. This is a read-only
attribute.
-
Node.previousSibling
- The node that immediately precedes this one with the same parent. For
instance the element with an end-tag that comes just before the self
element’s start-tag. Of course, XML documents are made up of more than just
elements so the previous sibling could be text, a comment, or something else.
If this node is the first child of the parent, this attribute will be
None. This is a read-only attribute.
-
Node.nextSibling
- The node that immediately follows this one with the same parent. See also
previousSibling. If this is the last child of the parent, this
attribute will be None. This is a read-only attribute.
-
Node.childNodes
- A list of nodes contained within this node. This is a read-only attribute.
-
Node.firstChild
- The first child of the node, if there are any, or None. This is a read-only
attribute.
-
Node.lastChild
- The last child of the node, if there are any, or None. This is a read-only
attribute.
-
Node.localName
- The part of the tagName following the colon if there is one, else the
entire tagName. The value is a string.
-
Node.prefix
- The part of the tagName preceding the colon if there is one, else the
empty string. The value is a string, or None
-
Node.namespaceURI
- The namespace associated with the element name. This will be a string or
None. This is a read-only attribute.
-
Node.nodeName
- This has a different meaning for each node type; see the DOM specification for
details. You can always get the information you would get here from another
property such as the tagName property for elements or the name
property for attributes. For all node types, the value of this attribute will be
either a string or None. This is a read-only attribute.
-
Node.nodeValue
- This has a different meaning for each node type; see the DOM specification for
details. The situation is similar to that with nodeName. The value is
a string or None.
-
Node.hasAttributes()
- Returns true if the node has any attributes.
-
Node.hasChildNodes()
- Returns true if the node has any child nodes.
-
Node.isSameNode(other)
Returns true if other refers to the same node as this node. This is especially
useful for DOM implementations which use any sort of proxy architecture (because
more than one object can refer to the same node).
Note
This is based on a proposed DOM Level 3 API which is still in the “working
draft” stage, but this particular interface appears uncontroversial. Changes
from the W3C will not necessarily affect this method in the Python DOM interface
(though any new W3C API for this would also be supported).
-
Node.appendChild(newChild)
- Add a new child node to this node at the end of the list of
children, returning newChild. If the node was already in
in the tree, it is removed first.
-
Node.insertBefore(newChild, refChild)
- Insert a new child node before an existing child. It must be the case that
refChild is a child of this node; if not, ValueError is raised.
newChild is returned. If refChild is None, it inserts newChild at the
end of the children’s list.
-
Node.removeChild(oldChild)
- Remove a child node. oldChild must be a child of this node; if not,
ValueError is raised. oldChild is returned on success. If oldChild
will not be used further, its unlink() method should be called.
-
Node.replaceChild(newChild, oldChild)
- Replace an existing node with a new node. It must be the case that oldChild
is a child of this node; if not, ValueError is raised.
-
Node.normalize()
Join adjacent text nodes so that all stretches of text are stored as single
Text instances. This simplifies processing text from a DOM tree for
many applications.
New in version 2.1.
-
Node.cloneNode(deep)
- Clone this node. Setting deep means to clone all child nodes as well. This
returns the clone.
NodeList Objects
A NodeList represents a sequence of nodes. These objects are used in
two ways in the DOM Core recommendation: the Element objects provides
one as its list of child nodes, and the getElementsByTagName() and
getElementsByTagNameNS() methods of Node return objects with this
interface to represent query results.
The DOM Level 2 recommendation defines one method and one attribute for these
objects:
-
NodeList.item(i)
- Return the i‘th item from the sequence, if there is one, or None. The
index i is not allowed to be less then zero or greater than or equal to the
length of the sequence.
-
NodeList.length
- The number of nodes in the sequence.
In addition, the Python DOM interface requires that some additional support is
provided to allow NodeList objects to be used as Python sequences. All
NodeList implementations must include support for __len__() and
__getitem__(); this allows iteration over the NodeList in
for statements and proper support for the len() built-in
function.
If a DOM implementation supports modification of the document, the
NodeList implementation must also support the __setitem__() and
__delitem__() methods.
DocumentType Objects
Information about the notations and entities declared by a document (including
the external subset if the parser uses it and can provide the information) is
available from a DocumentType object. The DocumentType for a
document is available from the Document object’s doctype
attribute; if there is no DOCTYPE declaration for the document, the
document’s doctype attribute will be set to None instead of an
instance of this interface.
DocumentType is a specialization of Node, and adds the
following attributes:
-
DocumentType.publicId
- The public identifier for the external subset of the document type definition.
This will be a string or None.
-
DocumentType.systemId
- The system identifier for the external subset of the document type definition.
This will be a URI as a string, or None.
-
DocumentType.internalSubset
- A string giving the complete internal subset from the document. This does not
include the brackets which enclose the subset. If the document has no internal
subset, this should be None.
-
DocumentType.name
- The name of the root element as given in the DOCTYPE declaration, if
present.
-
DocumentType.entities
- This is a NamedNodeMap giving the definitions of external entities.
For entity names defined more than once, only the first definition is provided
(others are ignored as required by the XML recommendation). This may be
None if the information is not provided by the parser, or if no entities are
defined.
-
DocumentType.notations
- This is a NamedNodeMap giving the definitions of notations. For
notation names defined more than once, only the first definition is provided
(others are ignored as required by the XML recommendation). This may be
None if the information is not provided by the parser, or if no notations
are defined.
Document Objects
A Document represents an entire XML document, including its constituent
elements, attributes, processing instructions, comments etc. Remember that it
inherits properties from Node.
-
Document.documentElement
- The one and only root element of the document.
-
Document.createElement(tagName)
- Create and return a new element node. The element is not inserted into the
document when it is created. You need to explicitly insert it with one of the
other methods such as insertBefore() or appendChild().
-
Document.createElementNS(namespaceURI, tagName)
- Create and return a new element with a namespace. The tagName may have a
prefix. The element is not inserted into the document when it is created. You
need to explicitly insert it with one of the other methods such as
insertBefore() or appendChild().
-
Document.createTextNode(data)
- Create and return a text node containing the data passed as a parameter. As
with the other creation methods, this one does not insert the node into the
tree.
- Create and return a comment node containing the data passed as a parameter. As
with the other creation methods, this one does not insert the node into the
tree.
-
Document.createProcessingInstruction(target, data)
- Create and return a processing instruction node containing the target and
data passed as parameters. As with the other creation methods, this one does
not insert the node into the tree.
-
Document.createAttribute(name)
- Create and return an attribute node. This method does not associate the
attribute node with any particular element. You must use
setAttributeNode() on the appropriate Element object to use the
newly created attribute instance.
-
Document.createAttributeNS(namespaceURI, qualifiedName)
- Create and return an attribute node with a namespace. The tagName may have a
prefix. This method does not associate the attribute node with any particular
element. You must use setAttributeNode() on the appropriate
Element object to use the newly created attribute instance.
-
Document.getElementsByTagName(tagName)
- Search for all descendants (direct children, children’s children, etc.) with a
particular element type name.
-
Document.getElementsByTagNameNS(namespaceURI, localName)
- Search for all descendants (direct children, children’s children, etc.) with a
particular namespace URI and localname. The localname is the part of the
namespace after the prefix.
Element Objects
Element is a subclass of Node, so inherits all the attributes
of that class.
-
Element.tagName
- The element type name. In a namespace-using document it may have colons in it.
The value is a string.
-
Element.getElementsByTagName(tagName)
- Same as equivalent method in the Document class.
-
Element.getElementsByTagNameNS(tagName)
- Same as equivalent method in the Document class.
-
Element.hasAttribute(name)
- Returns true if the element has an attribute named by name.
-
Element.hasAttributeNS(namespaceURI, localName)
- Returns true if the element has an attribute named by namespaceURI and
localName.
-
Element.getAttribute(name)
- Return the value of the attribute named by name as a string. If no such
attribute exists, an empty string is returned, as if the attribute had no value.
-
Element.getAttributeNode(attrname)
- Return the Attr node for the attribute named by attrname.
-
Element.getAttributeNS(namespaceURI, localName)
- Return the value of the attribute named by namespaceURI and localName as a
string. If no such attribute exists, an empty string is returned, as if the
attribute had no value.
-
Element.getAttributeNodeNS(namespaceURI, localName)
- Return an attribute value as a node, given a namespaceURI and localName.
-
Element.removeAttribute(name)
- Remove an attribute by name. If there is no matching attribute, a
NotFoundErr is raised.
-
Element.removeAttributeNode(oldAttr)
- Remove and return oldAttr from the attribute list, if present. If oldAttr is
not present, NotFoundErr is raised.
-
Element.removeAttributeNS(namespaceURI, localName)
- Remove an attribute by name. Note that it uses a localName, not a qname. No
exception is raised if there is no matching attribute.
-
Element.setAttribute(name, value)
- Set an attribute value from a string.
-
Element.setAttributeNode(newAttr)
- Add a new attribute node to the element, replacing an existing attribute if
necessary if the name attribute matches. If a replacement occurs, the
old attribute node will be returned. If newAttr is already in use,
InuseAttributeErr will be raised.
-
Element.setAttributeNodeNS(newAttr)
- Add a new attribute node to the element, replacing an existing attribute if
necessary if the namespaceURI and localName attributes match.
If a replacement occurs, the old attribute node will be returned. If newAttr
is already in use, InuseAttributeErr will be raised.
-
Element.setAttributeNS(namespaceURI, qname, value)
- Set an attribute value from a string, given a namespaceURI and a qname.
Note that a qname is the whole attribute name. This is different than above.
Attr Objects
Attr inherits from Node, so inherits all its attributes.
-
Attr.name
- The attribute name. In a namespace-using document it may have colons in it.
-
Attr.localName
- The part of the name following the colon if there is one, else the entire name.
This is a read-only attribute.
-
Attr.prefix
- The part of the name preceding the colon if there is one, else the empty string.
NamedNodeMap Objects
NamedNodeMap does not inherit from Node.
-
NamedNodeMap.length
- The length of the attribute list.
-
NamedNodeMap.item(index)
- Return an attribute with a particular index. The order you get the attributes
in is arbitrary but will be consistent for the life of a DOM. Each item is an
attribute node. Get its value with the value attribute.
There are also experimental methods that give this class more mapping behavior.
You can use them or you can use the standardized getAttribute*() family
of methods on the Element objects.
Text and CDATASection Objects
The Text interface represents text in the XML document. If the parser
and DOM implementation support the DOM’s XML extension, portions of the text
enclosed in CDATA marked sections are stored in CDATASection objects.
These two interfaces are identical, but provide different values for the
nodeType attribute.
These interfaces extend the Node interface. They cannot have child
nodes.
-
Text.data
- The content of the text node as a string.
Note
The use of a CDATASection node does not indicate that the node
represents a complete CDATA marked section, only that the content of the node
was part of a CDATA section. A single CDATA section may be represented by more
than one node in the document tree. There is no way to determine whether two
adjacent CDATASection nodes represent different CDATA marked sections.
ProcessingInstruction Objects
Represents a processing instruction in the XML document; this inherits from the
Node interface and cannot have child nodes.
-
ProcessingInstruction.target
- The content of the processing instruction up to the first whitespace character.
This is a read-only attribute.
-
ProcessingInstruction.data
- The content of the processing instruction following the first whitespace
character.
Exceptions
New in version 2.1.
The DOM Level 2 recommendation defines a single exception, DOMException,
and a number of constants that allow applications to determine what sort of
error occurred. DOMException instances carry a code attribute
that provides the appropriate value for the specific exception.
The Python DOM interface provides the constants, but also expands the set of
exceptions so that a specific exception exists for each of the exception codes
defined by the DOM. The implementations must raise the appropriate specific
exception, each of which carries the appropriate value for the code
attribute.
-
exception xml.dom.DOMException
- Base exception class used for all specific DOM exceptions. This exception class
cannot be directly instantiated.
-
exception xml.dom.DomstringSizeErr
- Raised when a specified range of text does not fit into a string. This is not
known to be used in the Python DOM implementations, but may be received from DOM
implementations not written in Python.
-
exception xml.dom.HierarchyRequestErr
- Raised when an attempt is made to insert a node where the node type is not
allowed.
-
exception xml.dom.IndexSizeErr
- Raised when an index or size parameter to a method is negative or exceeds the
allowed values.
-
exception xml.dom.InuseAttributeErr
- Raised when an attempt is made to insert an Attr node that is already
present elsewhere in the document.
-
exception xml.dom.InvalidAccessErr
- Raised if a parameter or an operation is not supported on the underlying object.
-
exception xml.dom.InvalidCharacterErr
- This exception is raised when a string parameter contains a character that is
not permitted in the context it’s being used in by the XML 1.0 recommendation.
For example, attempting to create an Element node with a space in the
element type name will cause this error to be raised.
-
exception xml.dom.InvalidModificationErr
- Raised when an attempt is made to modify the type of a node.
-
exception xml.dom.InvalidStateErr
- Raised when an attempt is made to use an object that is not defined or is no
longer usable.
-
exception xml.dom.NamespaceErr
- If an attempt is made to change any object in a way that is not permitted with
regard to the Namespaces in XML
recommendation, this exception is raised.
-
exception xml.dom.NotFoundErr
- Exception when a node does not exist in the referenced context. For example,
NamedNodeMap.removeNamedItem() will raise this if the node passed in does
not exist in the map.
-
exception xml.dom.NotSupportedErr
- Raised when the implementation does not support the requested type of object or
operation.
-
exception xml.dom.NoDataAllowedErr
- This is raised if data is specified for a node which does not support data.
-
exception xml.dom.NoModificationAllowedErr
- Raised on attempts to modify an object where modifications are not allowed (such
as for read-only nodes).
-
exception xml.dom.SyntaxErr
- Raised when an invalid or illegal string is specified.
-
exception xml.dom.WrongDocumentErr
- Raised when a node is inserted in a different document than it currently belongs
to, and the implementation does not support migrating the node from one document
to the other.
The exception codes defined in the DOM recommendation map to the exceptions
described above according to this table:
|