Doc/lib/xmldomminidom.tex

   1 \section{\module{xml.dom.minidom} ---
   2          Lightweight DOM implementation}
   3
   4 \declaremodule{standard}{xml.dom.minidom}
   5 \modulesynopsis{Lightweight Document Object Model (DOM) implementation.}
   6 \moduleauthor{Paul Prescod}{paul@prescod.net}
   7 \sectionauthor{Paul Prescod}{paul@prescod.net}
   8 \sectionauthor{Martin v. L\"owis}{loewis@informatik.hu-berlin.de}
   9
  10 \versionadded{2.0}
  11
  12 \module{xml.dom.minidom} is a light-weight implementation of the
  13 Document Object Model interface.  It is intended to be
  14 simpler than the full DOM and also significantly smaller.
  15
  16 DOM applications typically start by parsing some XML into a DOM.  With
  17 \module{xml.dom.minidom}, this is done through the parse functions:
  18
  19 \begin{verbatim}
  20 from xml.dom.minidom import parse, parseString
  21
  22 dom1 = parse('c:\\temp\\mydata.xml') # parse an XML file by name
  23
  24 datasource = open('c:\\temp\\mydata.xml')
  25 dom2 = parse(datasource)   # parse an open file
  26
  27 dom3 = parseString('<myxml>Some data<empty/> some more data</myxml>')
  28 \end{verbatim}
  29
  30 The parse function can take either a filename or an open file object.
  31
  32 \begin{funcdesc}{parse}{filename_or_file{, parser}}
  33   Return a \class{Document} from the given input. \var{filename_or_file}
  34   may be either a file name, or a file-like object. \var{parser}, if
  35   given, must be a SAX2 parser object. This function will change the
  36   document handler of the parser and activate namespace support; other
  37   parser configuration (like setting an entity resolver) must have been
  38   done in advance.
  39 \end{funcdesc}
  40
  41 If you have XML in a string, you can use the
  42 \function{parseString()} function instead:
  43
  44 \begin{funcdesc}{parseString}{string\optional{, parser}}
  45   Return a \class{Document} that represents the \var{string}. This
  46   method creates a \class{StringIO} object for the string and passes
  47   that on to \function{parse}.
  48 \end{funcdesc}
  49
  50 Both functions return a \class{Document} object representing the
  51 content of the document.
  52
  53 You can also create a \class{Document} node merely by instantiating a
  54 document object.  Then you could add child nodes to it to populate
  55 the DOM:
  56
  57 \begin{verbatim}
  58 from xml.dom.minidom import Document
  59
  60 newdoc = Document()
  61 newel = newdoc.createElement("some_tag")
  62 newdoc.appendChild(newel)
  63 \end{verbatim}
  64
  65 Once you have a DOM document object, you can access the parts of your
  66 XML document through its properties and methods.  These properties are
  67 defined in the DOM specification.  The main property of the document
  68 object is the \member{documentElement} property.  It gives you the
  69 main element in the XML document: the one that holds all others.  Here
  70 is an example program:
  71
  72 \begin{verbatim}
  73 dom3 = parseString("<myxml>Some data</myxml>")
  74 assert dom3.documentElement.tagName == "myxml"
  75 \end{verbatim}
  76
  77 When you are finished with a DOM, you should clean it up.  This is
  78 necessary because some versions of Python do not support garbage
  79 collection of objects that refer to each other in a cycle.  Until this
  80 restriction is removed from all versions of Python, it is safest to
  81 write your code as if cycles would not be cleaned up.
  82
  83 The way to clean up a DOM is to call its \method{unlink()} method:
  84
  85 \begin{verbatim}
  86 dom1.unlink()
  87 dom2.unlink()
  88 dom3.unlink()
  89 \end{verbatim}
  90
  91 \method{unlink()} is a \module{xml.dom.minidom}-specific extension to
  92 the DOM API.  After calling \method{unlink()} on a node, the node and
  93 its descendents are essentially useless.
  94
  95 \begin{seealso}
  96   \seetitle[http://www.w3.org/TR/REC-DOM-Level-1/]{Document Object
  97             Model (DOM) Level 1 Specification}
  98            {The W3C recommendation for the
  99             DOM supported by \module{xml.dom.minidom}.}
 100 \end{seealso}
 101
 102
 103 \subsection{DOM objects \label{dom-objects}}
 104
 105 The definition of the DOM API for Python is given as part of the
 106 \refmodule{xml.dom} module documentation.  This section lists the
 107 differences between the API and \refmodule{xml.dom.minidom}.
 108
 109
 110 \begin{methoddesc}{unlink}{}
 111 Break internal references within the DOM so that it will be garbage
 112 collected on versions of Python without cyclic GC.  Even when cyclic
 113 GC is available, using this can make large amounts of memory available
 114 sooner, so calling this on DOM objects as soon as they are no longer
 115 needed is good practice.  This only needs to be called on the
 116 \class{Document} object, but may be called on child nodes to discard
 117 children of that node.
 118 \end{methoddesc}
 119
 120 \begin{methoddesc}{writexml}{writer}
 121 Write XML to the writer object.  The writer should have a
 122 \method{write()} method which matches that of the file object
 123 interface.
 124 \end{methoddesc}
 125
 126 \begin{methoddesc}{toxml}{}
 127 Return the XML that the DOM represents as a string.
 128 \end{methoddesc}
 129
 130 The following standard DOM methods have special considerations with
 131 \refmodule{xml.dom.minidom}:
 132
 133 \begin{methoddesc}{cloneNode}{deep}
 134 Although this method was present in the version of
 135 \refmodule{xml.dom.minidom} packaged with Python 2.0, it was seriously
 136 broken.  This has been corrected for subsequent releases.
 137 \end{methoddesc}
 138
 139
 140 \subsection{DOM Example \label{dom-example}}
 141
 142 This example program is a fairly realistic example of a simple
 143 program. In this particular case, we do not take much advantage
 144 of the flexibility of the DOM.
 145
 146 \begin{verbatim}
 147 import xml.dom.minidom
 148
 149 document = """\
 150 <slideshow>
 151 <title>Demo slideshow</title>
 152 <slide><title>Slide title</title>
 153 <point>This is a demo</point>
 154 <point>Of a program for processing slides</point>
 155 </slide>
 156
 157 <slide><title>Another demo slide</title>
 158 <point>It is important</point>
 159 <point>To have more than</point>
 160 <point>one slide</point>
 161 </slide>
 162 </slideshow>
 163 """
 164
 165 dom = xml.dom.minidom.parseString(document)
 166
 167 space = " "
 168 def getText(nodelist):
 169     rc = ""
 170     for node in nodelist:
 171         if node.nodeType == node.TEXT_NODE:
 172             rc = rc + node.data
 173     return rc
 174
 175 def handleSlideshow(slideshow):
 176     print "<html>"
 177     handleSlideshowTitle(slideshow.getElementsByTagName("title")[0])
 178     slides = slideshow.getElementsByTagName("slide")
 179     handleToc(slides)
 180     handleSlides(slides)
 181     print "</html>"
 182
 183 def handleSlides(slides):
 184     for slide in slides:
 185        handleSlide(slide)
 186
 187 def handleSlide(slide):
 188     handleSlideTitle(slide.getElementsByTagName("title")[0])
 189     handlePoints(slide.getElementsByTagName("point"))
 190
 191 def handleSlideshowTitle(title):
 192     print "<title>%s</title>" % getText(title.childNodes)
 193
 194 def handleSlideTitle(title):
 195     print "<h2>%s</h2>" % getText(title.childNodes)
 196
 197 def handlePoints(points):
 198     print "<ul>"
 199     for point in points:
 200         handlePoint(point)
 201     print "</ul>"
 202
 203 def handlePoint(point):
 204     print "<li>%s</li>" % getText(point.childNodes)
 205
 206 def handleToc(slides):
 207     for slide in slides:
 208         title = slide.getElementsByTagName("title")[0]
 209         print "<p>%s</p>" % getText(title.childNodes)
 210
 211 handleSlideshow(dom)
 212 \end{verbatim}
 213
 214
 215 \subsection{minidom and the DOM standard \label{minidom-and-dom}}
 216
 217 The \refmodule{xml.dom.minidom} module is essentially a DOM
 218 1.0-compatible DOM with some DOM 2 features (primarily namespace
 219 features).
 220
 221 Usage of the DOM interface in Python is straight-forward.  The
 222 following mapping rules apply:
 223
 224 \begin{itemize}
 225 \item Interfaces are accessed through instance objects. Applications
 226       should not instantiate the classes themselves; they should use
 227       the creator functions available on the \class{Document} object.
 228       Derived interfaces support all operations (and attributes) from
 229       the base interfaces, plus any new operations.
 230
 231 \item Operations are used as methods. Since the DOM uses only
 232       \keyword{in} parameters, the arguments are passed in normal
 233       order (from left to right).   There are no optional
 234       arguments. \keyword{void} operations return \code{None}.
 235
 236 \item IDL attributes map to instance attributes. For compatibility
 237       with the OMG IDL language mapping for Python, an attribute
 238       \code{foo} can also be accessed through accessor methods
 239       \method{_get_foo()} and \method{_set_foo()}.  \keyword{readonly}
 240       attributes must not be changed; this is not enforced at
 241       runtime.
 242
 243 \item The types \code{short int}, \code{unsigned int}, \code{unsigned
 244       long long}, and \code{boolean} all map to Python integer
 245       objects.
 246
 247 \item The type \code{DOMString} maps to Python strings.
 248       \refmodule{xml.dom.minidom} supports either byte or Unicode
 249       strings, but will normally produce Unicode strings.  Attributes
 250       of type \code{DOMString} may also be \code{None}.
 251
 252 \item \keyword{const} declarations map to variables in their
 253       respective scope
 254       (e.g. \code{xml.dom.minidom.Node.PROCESSING_INSTRUCTION_NODE});
 255       they must not be changed.
 256
 257 \item \code{DOMException} is currently not supported in
 258       \refmodule{xml.dom.minidom}.  Instead,
 259       \refmodule{xml.dom.minidom} uses standard Python exceptions such
 260       as \exception{TypeError} and \exception{AttributeError}.
 261
 262 \item \class{NodeList} objects are implemented as Python's built-in
 263       list type, so don't support the official API, but are much more
 264       ``Pythonic.''
 265 \end{itemize}
 266
 267
 268 The following interfaces have no implementation in
 269 \refmodule{xml.dom.minidom}:
 270
 271 \begin{itemize}
 272 \item DOMTimeStamp
 273
 274 \item DocumentType (added in Python 2.1)
 275
 276 \item DOMImplementation (added in Python 2.1)
 277
 278 \item CharacterData
 279
 280 \item CDATASection
 281
 282 \item Notation
 283
 284 \item Entity
 285
 286 \item EntityReference
 287
 288 \item DocumentFragment
 289 \end{itemize}
 290
 291 Most of these reflect information in the XML document that is not of
 292 general utility to most DOM users.