20.6. 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.
더 보기
- 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.
20.6.1. 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 notNone
, imports the corresponding module and returns aDOMImplementation
object if the import succeeds. If no name is given, and if the environment variablePYTHON_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 thehasFeature()
method on availableDOMImplementation
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.
20.6.2. 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 |
---|---|---|
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Interface to the underlying implementation. |
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Base interface for most objects in a document. |
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Interface for a sequence of nodes. |
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Information about the declarations needed to process a document. |
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|
Object which represents an entire document. |
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Element nodes in the document hierarchy. |
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|
Attribute value nodes on element nodes. |
|
|
Representation of comments in the source document. |
|
|
Nodes containing textual content from the document. |
|
|
Processing instruction representation. |
An additional section describes the exceptions defined for working with the DOM in Python.
20.6.2.1. 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 childElement
object having the given namespaceUri and qualifiedName. The doctype must be aDocumentType
object created bycreateDocumentType()
, orNone
. In the Python DOM API, the first two arguments can also beNone
in order to indicate that noElement
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.
20.6.2.2. 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 aNode
object orNone
. ForElement
nodes, this will be the parent element, except for the root element, in which case it will be theDocument
object. ForAttr
nodes, this is alwaysNone
. This is a read-only attribute.
-
Node.
attributes
¶ A
NamedNodeMap
of attribute objects. Only elements have actual values for this; others provideNone
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 beNone
. 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 entiretagName
. 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, orNone
.
-
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 thename
property for attributes. For all node types, the value of this attribute will be either a string orNone
. 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 orNone
.
-
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).
참고
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 isNone
, 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, itsunlink()
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.
20.6.2.3. 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.
20.6.2.4. 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 beNone
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 beNone
if the information is not provided by the parser, or if no notations are defined.
20.6.2.5. 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()
orappendChild()
.
-
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()
orappendChild()
.
-
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 appropriateElement
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 appropriateElement
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.
20.6.2.6. 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
andlocalName
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.
20.6.2.7. 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.
20.6.2.8. 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.
20.6.2.9. 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.
20.6.2.10. 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.
참고
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.
20.6.2.11. 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.
20.6.2.12. 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:
Constant |
Exception |
---|---|
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20.6.3. 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.
20.6.3.1. Type Mapping¶
The IDL types used in the DOM specification are mapped to Python types according to the following table.
IDL Type |
Python Type |
---|---|
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20.6.3.2. 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.