:mod:`xml.dom` --- The Document Object Model API

================================================

.. module:: xml.dom

   :synopsis: Document Object Model API for Python.

.. sectionauthor:: Paul Prescod <paul@prescod.net>

.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>

.. versionadded:: 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.

.. XXX PyXML is dead...

.. The mapping of the Level 3 specification, currently

   only available in draft form, is being developed by the `Python XML Special

   Interest Group <http://www.python.org/sigs/xml-sig/>`_ as part of the `PyXML

   package <http://pyxml.sourceforge.net/>`_.  Refer to the documentation bundled

   with that package for information on the current state of DOM Level 3 support.

.. What if your needs are somewhere between SAX and the DOM?  Perhaps

   you cannot afford to load the entire tree in memory but you find the

   SAX model somewhat cumbersome and low-level.  There is also a module

   called xml.dom.pulldom that allows you to build trees of only the

   parts of a document that you need structured access to.  It also has

   features that allow you to find your way around the DOM.

   See http://www.prescod.net/python/pulldom

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 :class:`DOMImplementation` object class which

provides access to :class:`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

:class:`Document` object.  In Python, each DOM implementation will provide a

function :func:`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

:ref:`dom-conformance` for a detailed discussion of mapping requirements.

.. seealso::

   `Document Object Model (DOM) Level 2 Specification <http://www.w3.org/TR/DOM-Level-2-Core/>`_

      The W3C recommendation upon which the Python DOM API is based.

   `Document Object Model (DOM) Level 1 Specification <http://www.w3.org/TR/REC-DOM-Level-1/>`_

      The W3C recommendation for the DOM supported by :mod:`xml.dom.minidom`.

   `Python Language Mapping Specification <http://www.omg.org/docs/formal/02-11-05.pdf>`_

      This specifies the mapping from OMG IDL to Python.

Module Contents

---------------

The :mod:`xml.dom` contains the following functions:

.. function:: registerDOMImplementation(name, factory)

   Register the *factory* function with the name *name*.  The factory function

   should return an object which implements the :class:`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).

.. function:: 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 :class:`DOMImplementation` object if the

   import succeeds.  If no name is given, and if the environment variable

   :envvar:`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

   :exc:`ImportError`.  The features list must be a sequence of ``(feature,

   version)`` pairs which are passed to the :meth:`hasFeature` method on available

   :class:`DOMImplementation` objects.

Some convenience constants are also provided:

.. data:: EMPTY_NAMESPACE

   The value used to indicate that no namespace is associated with a node in the

   DOM.  This is typically found as the :attr:`namespaceURI` of a node, or used as

   the *namespaceURI* parameter to a namespaces-specific method.

   .. versionadded:: 2.2

.. data:: XML_NAMESPACE

   The namespace URI associated with the reserved prefix ``xml``, as defined by

   `Namespaces in XML <http://www.w3.org/TR/REC-xml-names/>`_ (section 4).

   .. versionadded:: 2.2

.. data:: XMLNS_NAMESPACE

   The namespace URI for namespace declarations, as defined by `Document Object

   Model (DOM) Level 2 Core Specification

   <http://www.w3.org/TR/DOM-Level-2-Core/core.html>`_ (section 1.1.8).

   .. versionadded:: 2.2

.. data:: XHTML_NAMESPACE

   The URI of the XHTML namespace as defined by `XHTML 1.0: The Extensible

   HyperText Markup Language <http://www.w3.org/TR/xhtml1/>`_ (section 3.1.1).

   .. versionadded:: 2.2

In addition, :mod:`xml.dom` contains a base :class:`Node` class and the DOM

exception classes.  The :class:`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 :class:`Node` class

provided as part of this module does provide the constants used for the

:attr:`nodeType` attribute on concrete :class:`Node` objects; they are located

within the class rather than at the module level to conform with the DOM

specifications.

.. Should the Node documentation go here?

.. _dom-objects:

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.

+--------------------------------+-----------------------------------+---------------------------------+

| Interface                      | Section                           | Purpose                         |

+================================+===================================+=================================+

| :class:`DOMImplementation`     | :ref:`dom-implementation-objects` | Interface to the underlying     |

|                                |                                   | implementation.                 |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`Node`                  | :ref:`dom-node-objects`           | Base interface for most objects |

|                                |                                   | in a document.                  |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`NodeList`              | :ref:`dom-nodelist-objects`       | Interface for a sequence of     |

|                                |                                   | nodes.                          |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`DocumentType`          | :ref:`dom-documenttype-objects`   | Information about the           |

|                                |                                   | declarations needed to process  |

|                                |                                   | a document.                     |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`Document`              | :ref:`dom-document-objects`       | Object which represents an      |

|                                |                                   | entire document.                |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`Element`               | :ref:`dom-element-objects`        | Element nodes in the document   |

|                                |                                   | hierarchy.                      |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`Attr`                  | :ref:`dom-attr-objects`           | Attribute value nodes on        |

|                                |                                   | element nodes.                  |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`Comment`               | :ref:`dom-comment-objects`        | Representation of comments in   |

|                                |                                   | the source document.            |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`Text`                  | :ref:`dom-text-objects`           | Nodes containing textual        |

|                                |                                   | content from the document.      |

+--------------------------------+-----------------------------------+---------------------------------+

| :class:`ProcessingInstruction` | :ref:`dom-pi-objects`             | Processing instruction          |

|                                |                                   | representation.                 |

+--------------------------------+-----------------------------------+---------------------------------+

An additional section describes the exceptions defined for working with the DOM

in Python.

.. _dom-implementation-objects:

DOMImplementation Objects

^^^^^^^^^^^^^^^^^^^^^^^^^

The :class:`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 :class:`Document` and

:class:`DocumentType` objects using the :class:`DOMImplementation` as well.

.. method:: DOMImplementation.hasFeature(feature, version)

   Return true if the feature identified by the pair of strings *feature* and

   *version* is implemented.

.. method:: DOMImplementation.createDocument(namespaceUri, qualifiedName, doctype)

   Return a new :class:`Document` object (the root of the DOM), with a child

   :class:`Element` object having the given *namespaceUri* and *qualifiedName*. The

   *doctype* must be a :class:`DocumentType` object created by

   :meth:`createDocumentType`, or ``None``. In the Python DOM API, the first two

   arguments can also be ``None`` in order to indicate that no :class:`Element`

   child is to be created.

.. method:: DOMImplementation.createDocumentType(qualifiedName, publicId, systemId)

   Return a new :class:`DocumentType` object that encapsulates the given

   *qualifiedName*, *publicId*, and *systemId* strings, representing the

   information contained in an XML document type declaration.

.. _dom-node-objects:

Node Objects

^^^^^^^^^^^^

All of the components of an XML document are subclasses of :class:`Node`.

.. attribute:: Node.nodeType

   An integer representing the node type.  Symbolic constants for the types are on

   the :class:`Node` object: :const:`ELEMENT_NODE`, :const:`ATTRIBUTE_NODE`,

   :const:`TEXT_NODE`, :const:`CDATA_SECTION_NODE`, :const:`ENTITY_NODE`,

   :const:`PROCESSING_INSTRUCTION_NODE`, :const:`COMMENT_NODE`,

   :const:`DOCUMENT_NODE`, :const:`DOCUMENT_TYPE_NODE`, :const:`NOTATION_NODE`.

   This is a read-only attribute.

.. attribute:: Node.parentNode

   The parent of the current node, or ``None`` for the document node. The value is

   always a :class:`Node` object or ``None``.  For :class:`Element` nodes, this

   will be the parent element, except for the root element, in which case it will

   be the :class:`Document` object. For :class:`Attr` nodes, this is always

   ``None``. This is a read-only attribute.

.. attribute:: Node.attributes

   A :class:`NamedNodeMap` of attribute objects.  Only elements have actual values

   for this; others provide ``None`` for this attribute. This is a read-only

   attribute.

.. 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.

.. attribute:: Node.nextSibling

   The node that immediately follows this one with the same parent.  See also

   :attr:`previousSibling`.  If this is the last child of the parent, this

   attribute will be ``None``. This is a read-only attribute.

.. attribute:: Node.childNodes

   A list of nodes contained within this node. This is a read-only attribute.

.. attribute:: Node.firstChild

   The first child of the node, if there are any, or ``None``. This is a read-only

   attribute.

.. attribute:: Node.lastChild

   The last child of the node, if there are any, or ``None``. This is a read-only

   attribute.

.. attribute:: Node.localName

   The part of the :attr:`tagName` following the colon if there is one, else the

   entire :attr:`tagName`.  The value is a string.

.. attribute:: Node.prefix

   The part of the :attr:`tagName` preceding the colon if there is one, else the

   empty string.  The value is a string, or ``None``

.. attribute:: Node.namespaceURI

   The namespace associated with the element name.  This will be a string or

   ``None``.  This is a read-only attribute.

.. 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 :attr:`tagName` property for elements or the :attr:`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.

.. 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 :attr:`nodeName`.  The value is

   a string or ``None``.

.. method:: Node.hasAttributes()

   Returns true if the node has any attributes.

.. method:: Node.hasChildNodes()

   Returns true if the node has any child nodes.

.. method:: 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).

.. method:: 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.

.. method:: 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, :exc:`ValueError` is raised.

   *newChild* is returned. If *refChild* is ``None``, it inserts *newChild* at the

   end of the children's list.

.. method:: Node.removeChild(oldChild)

   Remove a child node.  *oldChild* must be a child of this node; if not,

   :exc:`ValueError` is raised.  *oldChild* is returned on success.  If *oldChild*

   will not be used further, its :meth:`unlink` method should be called.

.. method:: 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, :exc:`ValueError` is raised.

.. method:: Node.normalize()

   Join adjacent text nodes so that all stretches of text are stored as single

   :class:`Text` instances.  This simplifies processing text from a DOM tree for

   many applications.

   .. versionadded:: 2.1

.. method:: Node.cloneNode(deep)

   Clone this node.  Setting *deep* means to clone all child nodes as well.  This

   returns the clone.

.. _dom-nodelist-objects:

NodeList Objects

^^^^^^^^^^^^^^^^

A :class:`NodeList` represents a sequence of nodes.  These objects are used in

two ways in the DOM Core recommendation:  the :class:`Element` objects provides

one as its list of child nodes, and the :meth:`getElementsByTagName` and

:meth:`getElementsByTagNameNS` methods of :class:`Node` return objects with this

interface to represent query results.

The DOM Level 2 recommendation defines one method and one attribute for these

objects:

.. method:: 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.

.. attribute:: NodeList.length

   The number of nodes in the sequence.

In addition, the Python DOM interface requires that some additional support is

provided to allow :class:`NodeList` objects to be used as Python sequences.  All

:class:`NodeList` implementations must include support for :meth:`__len__` and

:meth:`__getitem__`; this allows iteration over the :class:`NodeList` in

:keyword:`for` statements and proper support for the :func:`len` built-in

function.

If a DOM implementation supports modification of the document, the

:class:`NodeList` implementation must also support the :meth:`__setitem__` and

:meth:`__delitem__` methods.

.. _dom-documenttype-objects:

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 :class:`DocumentType` object.  The :class:`DocumentType` for a

document is available from the :class:`Document` object's :attr:`doctype`

attribute; if there is no ``DOCTYPE`` declaration for the document, the

document's :attr:`doctype` attribute will be set to ``None`` instead of an

instance of this interface.

:class:`DocumentType` is a specialization of :class:`Node`, and adds the

following attributes:

.. attribute:: DocumentType.publicId

   The public identifier for the external subset of the document type definition.

   This will be a string or ``None``.

.. attribute:: DocumentType.systemId

   The system identifier for the external subset of the document type definition.

   This will be a URI as a string, or ``None``.

.. attribute:: 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``.

.. attribute:: DocumentType.name

   The name of the root element as given in the ``DOCTYPE`` declaration, if

   present.

.. attribute:: DocumentType.entities

   This is a :class:`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.

.. attribute:: DocumentType.notations

   This is a :class:`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.

.. _dom-document-objects:

Document Objects

^^^^^^^^^^^^^^^^

A :class:`Document` represents an entire XML document, including its constituent

elements, attributes, processing instructions, comments etc.  Remember that it

inherits properties from :class:`Node`.

.. attribute:: Document.documentElement

   The one and only root element of the document.