274 lines
10 KiB
ReStructuredText
274 lines
10 KiB
ReStructuredText
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:mod:`xml.dom.minidom` --- Lightweight DOM implementation
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=========================================================
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.. module:: xml.dom.minidom
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:synopsis: Lightweight Document Object Model (DOM) implementation.
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.. moduleauthor:: Paul Prescod <paul@prescod.net>
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.. sectionauthor:: Paul Prescod <paul@prescod.net>
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.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
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.. versionadded:: 2.0
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:mod:`xml.dom.minidom` is a light-weight implementation of the Document Object
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Model interface. It is intended to be simpler than the full DOM and also
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significantly smaller.
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DOM applications typically start by parsing some XML into a DOM. With
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:mod:`xml.dom.minidom`, this is done through the parse functions::
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from xml.dom.minidom import parse, parseString
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dom1 = parse('c:\\temp\\mydata.xml') # parse an XML file by name
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datasource = open('c:\\temp\\mydata.xml')
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dom2 = parse(datasource) # parse an open file
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dom3 = parseString('<myxml>Some data<empty/> some more data</myxml>')
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The :func:`parse` function can take either a filename or an open file object.
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.. function:: parse(filename_or_file, parser)
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Return a :class:`Document` from the given input. *filename_or_file* may be
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either a file name, or a file-like object. *parser*, if given, must be a SAX2
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parser object. This function will change the document handler of the parser and
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activate namespace support; other parser configuration (like setting an entity
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resolver) must have been done in advance.
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If you have XML in a string, you can use the :func:`parseString` function
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instead:
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.. function:: parseString(string[, parser])
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Return a :class:`Document` that represents the *string*. This method creates a
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:class:`StringIO` object for the string and passes that on to :func:`parse`.
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Both functions return a :class:`Document` object representing the content of the
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document.
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What the :func:`parse` and :func:`parseString` functions do is connect an XML
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parser with a "DOM builder" that can accept parse events from any SAX parser and
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convert them into a DOM tree. The name of the functions are perhaps misleading,
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but are easy to grasp when learning the interfaces. The parsing of the document
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will be completed before these functions return; it's simply that these
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functions do not provide a parser implementation themselves.
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You can also create a :class:`Document` by calling a method on a "DOM
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Implementation" object. You can get this object either by calling the
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:func:`getDOMImplementation` function in the :mod:`xml.dom` package or the
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:mod:`xml.dom.minidom` module. Using the implementation from the
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:mod:`xml.dom.minidom` module will always return a :class:`Document` instance
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from the minidom implementation, while the version from :mod:`xml.dom` may
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provide an alternate implementation (this is likely if you have the `PyXML
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package <http://pyxml.sourceforge.net/>`_ installed). Once you have a
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:class:`Document`, you can add child nodes to it to populate the DOM::
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from xml.dom.minidom import getDOMImplementation
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impl = getDOMImplementation()
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newdoc = impl.createDocument(None, "some_tag", None)
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top_element = newdoc.documentElement
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text = newdoc.createTextNode('Some textual content.')
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top_element.appendChild(text)
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Once you have a DOM document object, you can access the parts of your XML
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document through its properties and methods. These properties are defined in
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the DOM specification. The main property of the document object is the
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:attr:`documentElement` property. It gives you the main element in the XML
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document: the one that holds all others. Here is an example program::
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dom3 = parseString("<myxml>Some data</myxml>")
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assert dom3.documentElement.tagName == "myxml"
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When you are finished with a DOM, you should clean it up. This is necessary
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because some versions of Python do not support garbage collection of objects
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that refer to each other in a cycle. Until this restriction is removed from all
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versions of Python, it is safest to write your code as if cycles would not be
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cleaned up.
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The way to clean up a DOM is to call its :meth:`unlink` method::
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dom1.unlink()
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dom2.unlink()
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dom3.unlink()
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:meth:`unlink` is a :mod:`xml.dom.minidom`\ -specific extension to the DOM API.
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After calling :meth:`unlink` on a node, the node and its descendants are
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essentially useless.
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.. seealso::
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`Document Object Model (DOM) Level 1 Specification <http://www.w3.org/TR/REC-DOM-Level-1/>`_
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The W3C recommendation for the DOM supported by :mod:`xml.dom.minidom`.
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.. _minidom-objects:
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DOM Objects
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-----------
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The definition of the DOM API for Python is given as part of the :mod:`xml.dom`
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module documentation. This section lists the differences between the API and
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:mod:`xml.dom.minidom`.
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.. method:: Node.unlink()
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Break internal references within the DOM so that it will be garbage collected on
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versions of Python without cyclic GC. Even when cyclic GC is available, using
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this can make large amounts of memory available sooner, so calling this on DOM
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objects as soon as they are no longer needed is good practice. This only needs
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to be called on the :class:`Document` object, but may be called on child nodes
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to discard children of that node.
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.. method:: Node.writexml(writer[,indent=""[,addindent=""[,newl=""]]])
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Write XML to the writer object. The writer should have a :meth:`write` method
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which matches that of the file object interface. The *indent* parameter is the
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indentation of the current node. The *addindent* parameter is the incremental
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indentation to use for subnodes of the current one. The *newl* parameter
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specifies the string to use to terminate newlines.
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.. versionchanged:: 2.1
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The optional keyword parameters *indent*, *addindent*, and *newl* were added to
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support pretty output.
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.. versionchanged:: 2.3
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For the :class:`Document` node, an additional keyword argument
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*encoding* can be used to specify the encoding field of the XML header.
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.. method:: Node.toxml([encoding])
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Return the XML that the DOM represents as a string.
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With no argument, the XML header does not specify an encoding, and the result is
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Unicode string if the default encoding cannot represent all characters in the
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document. Encoding this string in an encoding other than UTF-8 is likely
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incorrect, since UTF-8 is the default encoding of XML.
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With an explicit *encoding* [1]_ argument, the result is a byte string in the
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specified encoding. It is recommended that this argument is always specified. To
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avoid :exc:`UnicodeError` exceptions in case of unrepresentable text data, the
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encoding argument should be specified as "utf-8".
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.. versionchanged:: 2.3
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the *encoding* argument was introduced.
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.. method:: Node.toprettyxml([indent[, newl]])
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Return a pretty-printed version of the document. *indent* specifies the
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indentation string and defaults to a tabulator; *newl* specifies the string
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emitted at the end of each line and defaults to ``\n``.
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.. versionadded:: 2.1
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.. versionchanged:: 2.3
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the encoding argument; see :meth:`toxml`.
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The following standard DOM methods have special considerations with
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:mod:`xml.dom.minidom`:
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.. method:: Node.cloneNode(deep)
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Although this method was present in the version of :mod:`xml.dom.minidom`
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packaged with Python 2.0, it was seriously broken. This has been corrected for
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subsequent releases.
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.. _dom-example:
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DOM Example
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-----------
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This example program is a fairly realistic example of a simple program. In this
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particular case, we do not take much advantage of the flexibility of the DOM.
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.. literalinclude:: ../includes/minidom-example.py
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.. _minidom-and-dom:
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minidom and the DOM standard
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----------------------------
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The :mod:`xml.dom.minidom` module is essentially a DOM 1.0-compatible DOM with
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some DOM 2 features (primarily namespace features).
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Usage of the DOM interface in Python is straight-forward. The following mapping
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rules apply:
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* Interfaces are accessed through instance objects. Applications should not
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instantiate the classes themselves; they should use the creator functions
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available on the :class:`Document` object. Derived interfaces support all
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operations (and attributes) from the base interfaces, plus any new operations.
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* Operations are used as methods. Since the DOM uses only :keyword:`in`
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parameters, the arguments are passed in normal order (from left to right).
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There are no optional arguments. ``void`` operations return ``None``.
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* IDL attributes map to instance attributes. For compatibility with the OMG IDL
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language mapping for Python, an attribute ``foo`` can also be accessed through
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accessor methods :meth:`_get_foo` and :meth:`_set_foo`. ``readonly``
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attributes must not be changed; this is not enforced at runtime.
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* The types ``short int``, ``unsigned int``, ``unsigned long long``, and
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``boolean`` all map to Python integer objects.
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* The type ``DOMString`` maps to Python strings. :mod:`xml.dom.minidom` supports
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either byte or Unicode strings, but will normally produce Unicode strings.
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Values of type ``DOMString`` may also be ``None`` where allowed to have the IDL
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``null`` value by the DOM specification from the W3C.
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* ``const`` declarations map to variables in their respective scope (e.g.
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``xml.dom.minidom.Node.PROCESSING_INSTRUCTION_NODE``); they must not be changed.
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* ``DOMException`` is currently not supported in :mod:`xml.dom.minidom`.
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Instead, :mod:`xml.dom.minidom` uses standard Python exceptions such as
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:exc:`TypeError` and :exc:`AttributeError`.
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* :class:`NodeList` objects are implemented using Python's built-in list type.
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Starting with Python 2.2, these objects provide the interface defined in the DOM
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specification, but with earlier versions of Python they do not support the
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official API. They are, however, much more "Pythonic" than the interface
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defined in the W3C recommendations.
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The following interfaces have no implementation in :mod:`xml.dom.minidom`:
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* :class:`DOMTimeStamp`
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* :class:`DocumentType` (added in Python 2.1)
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* :class:`DOMImplementation` (added in Python 2.1)
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* :class:`CharacterData`
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* :class:`CDATASection`
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* :class:`Notation`
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* :class:`Entity`
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* :class:`EntityReference`
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* :class:`DocumentFragment`
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Most of these reflect information in the XML document that is not of general
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utility to most DOM users.
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.. rubric:: Footnotes
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.. [#] The encoding string included in XML output should conform to the
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appropriate standards. For example, "UTF-8" is valid, but "UTF8" is
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not. See http://www.w3.org/TR/2006/REC-xml11-20060816/#NT-EncodingDecl
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and http://www.iana.org/assignments/character-sets .
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