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Python 3.6.5 Documentation > "xml.dom" — The Document Object Model API

"xml.dom" — The Document Object Model API
*****************************************

**Source code:** Lib/xml/dom/__init__.py

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

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.

See also:

Document Object Model (DOM) Level 2 Specification
The W3C recommendation upon which the Python DOM API is based.

Document Object Model (DOM) Level 1 Specification
The W3C recommendation for the DOM supported by
"xml.dom.minidom".

Python Language Mapping Specification
This specifies the mapping from OMG IDL to Python.

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=None, 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.

xml.dom.XML_NAMESPACE

The namespace URI associated with the reserved prefix "xml", as
defined by Namespaces in XML (section 4).

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).

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).

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 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.

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: an "Element" object
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 than 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.

Document.createComment(data)

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(namespaceURI, localName)

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 include a
colon.

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.

Attr.value

The text value of the attribute. This is a synonym for the
"nodeValue" attribute.

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.

Comment Objects
---------------

"Comment" represents a comment in the XML document. It is a subclass
of "Node", but cannot have child nodes.

Comment.data

The content of the comment as a string. The attribute contains all
characters between the leading "<!-""-" and trailing "-""->", but
does not include them.

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
----------

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:

Conformance
===========

This section describes the conformance requirements and relationships
between the Python DOM API, the W3C DOM recommendations, and the OMG
IDL mapping for Python.

Type Mapping
------------

The IDL types used in the DOM specification are mapped to Python types
according to the following table.

+--------------------+---------------------------------------------+
| IDL Type | Python Type |
+====================+=============================================+
| "boolean" | "bool" or "int" |
+--------------------+---------------------------------------------+
| "int" | "int" |
+--------------------+---------------------------------------------+
| "long int" | "int" |
+--------------------+---------------------------------------------+
| "unsigned int" | "int" |
+--------------------+---------------------------------------------+
| "DOMString" | "str" or "bytes" |
+--------------------+---------------------------------------------+
| "null" | "None" |
+--------------------+---------------------------------------------+

Accessor Methods
----------------

The mapping from OMG IDL to Python defines accessor functions for IDL
"attribute" declarations in much the way the Java mapping does.
Mapping the IDL declarations

readonly attribute string someValue;
attribute string anotherValue;

yields three accessor functions: a “get” method for "someValue"
("_get_someValue()"), and “get” and “set” methods for "anotherValue"
("_get_anotherValue()" and "_set_anotherValue()"). The mapping, in
particular, does not require that the IDL attributes are accessible as
normal Python attributes: "object.someValue" is *not* required to
work, and may raise an "AttributeError".

The Python DOM API, however, *does* require that normal attribute
access work. This means that the typical surrogates generated by
Python IDL compilers are not likely to work, and wrapper objects may
be needed on the client if the DOM objects are accessed via CORBA.
While this does require some additional consideration for CORBA DOM
clients, the implementers with experience using DOM over CORBA from
Python do not consider this a problem. Attributes that are declared
"readonly" may not restrict write access in all DOM implementations.

In the Python DOM API, accessor functions are not required. If
provided, they should take the form defined by the Python IDL mapping,
but these methods are considered unnecessary since the attributes are
accessible directly from Python. “Set” accessors should never be
provided for "readonly" attributes.

The IDL definitions do not fully embody the requirements of the W3C
DOM API, such as the notion of certain objects, such as the return
value of "getElementsByTagName()", being “live”. The Python DOM API
does not require implementations to enforce such requirements.

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