doc/html/element_classes.html

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 to:</a></li></ul></li></ul></li><li><a href="http://lxml.de/sitemap.html">Sitemap</a></li></ul></div><h1 class="title">Using custom Element classes in lxml</h1>
  12
  13 <p>lxml has very sophisticated support for custom Element classes.  You
  14 can provide your own classes for Elements and have lxml use them by
  15 default for all elements generated by a specific parser, only for a
  16 specific tag name in a specific namespace or even for an exact element
  17 at a specific position in the tree.</p>
  18 <p>Custom Elements must inherit from the <tt class="docutils literal">lxml.etree.ElementBase</tt> class, which
  19 provides the Element interface for subclasses:</p>
  20 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">lxml</span> <span class="kn">import</span> <span class="n">etree</span>
  21
  22 <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">honk</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">ElementBase</span><span class="p">):</span>
  23 <span class="gp">... </span>   <span class="nd">@property</span>
  24 <span class="gp">... </span>   <span class="k">def</span> <span class="nf">honking</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
  25 <span class="gp">... </span>      <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'honking'</span><span class="p">)</span> <span class="o">==</span> <span class="s">'true'</span>
  26 </pre></div>
  27 <p>This defines a new Element class <tt class="docutils literal">honk</tt> with a property <tt class="docutils literal">honking</tt>.</p>
  28 <p>The following document describes how you can make lxml.etree use these
  29 custom Element classes.</p>
  30 <div class="contents topic" id="contents">
  31 <p class="topic-title first">Contents</p>
  32 <ul class="simple">
  33 <li><a class="reference internal" href="#background-on-element-proxies" id="id2">Background on Element proxies</a></li>
  34 <li><a class="reference internal" href="#element-initialization" id="id3">Element initialization</a></li>
  35 <li><a class="reference internal" href="#setting-up-a-class-lookup-scheme" id="id4">Setting up a class lookup scheme</a><ul>
  36 <li><a class="reference internal" href="#default-class-lookup" id="id5">Default class lookup</a></li>
  37 <li><a class="reference internal" href="#namespace-class-lookup" id="id6">Namespace class lookup</a></li>
  38 <li><a class="reference internal" href="#attribute-based-lookup" id="id7">Attribute based lookup</a></li>
  39 <li><a class="reference internal" href="#custom-element-class-lookup" id="id8">Custom element class lookup</a></li>
  40 <li><a class="reference internal" href="#tree-based-element-class-lookup-in-python" id="id9">Tree based element class lookup in Python</a></li>
  41 </ul>
  42 </li>
  43 <li><a class="reference internal" href="#generating-xml-with-custom-classes" id="id10">Generating XML with custom classes</a></li>
  44 <li><a class="reference internal" href="#id1" id="id11">Implementing namespaces</a></li>
  45 </ul>
  46 </div>
  47 <div class="section" id="background-on-element-proxies">
  48 <h1>Background on Element proxies</h1>
  49 <p>Being based on libxml2, lxml.etree holds the entire XML tree in a C
  50 structure.  To communicate with Python code, it creates Python proxy
  51 objects for the XML elements on demand.</p>
  52 <blockquote>
  53 <img alt="proxies.png" src="proxies.png" />
  54 </blockquote>
  55 <p>The mapping between C elements and Python Element classes is
  56 completely configurable.  When you ask lxml.etree for an Element by
  57 using its API, it will instantiate your classes for you.  All you have
  58 to do is tell lxml which class to use for which kind of Element.  This
  59 is done through a class lookup scheme, as described in the sections
  60 below.</p>
  61 </div>
  62 <div class="section" id="element-initialization">
  63 <h1>Element initialization</h1>
  64 <p>There is one thing to know up front.  Element classes <em>must not</em> have
  65 an <tt class="docutils literal">__init___</tt> or <tt class="docutils literal">__new__</tt> method.  There should not be any
  66 internal state either, except for the data stored in the underlying
  67 XML tree.  Element instances are created and garbage collected at
  68 need, so there is normally no way to predict when and how often a
  69 proxy is created for them.  Even worse, when the <tt class="docutils literal">__init__</tt> method
  70 is called, the object is not even initialized yet to represent the XML
  71 tag, so there is not much use in providing an <tt class="docutils literal">__init__</tt> method in
  72 subclasses.</p>
  73 <p>Most use cases will not require any class initialisation or proxy
  74 state, so you can content yourself with skipping to the next section
  75 for now.  However, if you really need to set up your element class on
  76 instantiation, or need a way to persistently store state in the proxy
  77 instances instead of the XML tree, here is a way to do so.</p>
  78 <p>There is one important guarantee regarding Element proxies.  Once a
  79 proxy has been instantiated, it will keep alive as long as there is a
  80 Python reference to it, and any access to the XML element in the tree
  81 will return this very instance.  Therefore, if you need to store local
  82 state in a custom Element class (which is generally discouraged), you
  83 can do so by keeping the Elements in a tree alive.  If the tree
  84 doesn't change, you can simply do this:</p>
  85 <div class="syntax"><pre><span class="n">proxy_cache</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">root</span><span class="o">.</span><span class="n">iter</span><span class="p">())</span>
  86 </pre></div>
  87 <p>or</p>
  88 <div class="syntax"><pre><span class="n">proxy_cache</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="n">root</span><span class="o">.</span><span class="n">iter</span><span class="p">())</span>
  89 </pre></div>
  90 <p>or use any other suitable container.  Note that you have to keep this
  91 cache manually up to date if the tree changes, which can get tricky in
  92 cases.</p>
  93 <p>For proxy initialisation, ElementBase classes have an <tt class="docutils literal">_init()</tt>
  94 method that can be overridden, as oppose to the normal <tt class="docutils literal">__init__()</tt>
  95 method.  It can be used to modify the XML tree, e.g. to construct
  96 special children or verify and update attributes.</p>
  97 <p>The semantics of <tt class="docutils literal">_init()</tt> are as follows:</p>
  98 <ul class="simple">
  99 <li>It is called once on Element class instantiation time.  That is,
 100 when a Python representation of the element is created by lxml.  At
 101 that time, the element object is completely initialized to represent
 102 a specific XML element within the tree.</li>
 103 <li>The method has complete access to the XML tree.  Modifications can be done
 104 in exactly the same way as anywhere else in the program.</li>
 105 <li>Python representations of elements may be created multiple times during the
 106 lifetime of an XML element in the underlying C tree.  The <tt class="docutils literal">_init()</tt> code
 107 provided by subclasses must take special care by itself that multiple
 108 executions either are harmless or that they are prevented by some kind of
 109 flag in the XML tree.  The latter can be achieved by modifying an attribute
 110 value or by removing or adding a specific child node and then verifying this
 111 before running through the init process.</li>
 112 <li>Any exceptions raised in <tt class="docutils literal">_init()</tt> will be propagated throught the API
 113 call that lead to the creation of the Element.  So be careful with the code
 114 you write here as its exceptions may turn up in various unexpected places.</li>
 115 </ul>
 116 </div>
 117 <div class="section" id="setting-up-a-class-lookup-scheme">
 118 <h1>Setting up a class lookup scheme</h1>
 119 <p>The first thing to do when deploying custom element classes is to register a
 120 class lookup scheme on a parser.  lxml.etree provides quite a number of
 121 different schemes that also support class lookup based on namespaces or
 122 attribute values.  Most lookups support fallback chaining, which allows the
 123 next lookup mechanism to take over when the previous one fails to find a
 124 class.</p>
 125 <p>For example, setting the <tt class="docutils literal">honk</tt> Element as a default element class
 126 for a parser works as follows:</p>
 127 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">parser_lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementDefaultClassLookup</span><span class="p">(</span><span class="n">element</span><span class="o">=</span><span class="n">honk</span><span class="p">)</span>
 128 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 129 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">parser_lookup</span><span class="p">)</span>
 130 </pre></div>
 131 <p>There is one drawback of the parser based scheme: the <tt class="docutils literal">Element()</tt> factory
 132 does not know about your specialised parser and creates a new document that
 133 deploys the default parser:</p>
 134 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">Element</span><span class="p">(</span><span class="s">"root"</span><span class="p">)</span>
 135 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">el</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 136 <span class="go">False</span>
 137 </pre></div>
 138 <p>You should therefore avoid using this factory function in code that
 139 uses custom classes.  The <tt class="docutils literal">makeelement()</tt> method of parsers provides
 140 a simple replacement:</p>
 141 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">makeelement</span><span class="p">(</span><span class="s">"root"</span><span class="p">)</span>
 142 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">el</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 143 <span class="go">True</span>
 144 </pre></div>
 145 <p>If you use a parser at the module level, you can easily redirect a module
 146 level <tt class="docutils literal">Element()</tt> factory to the parser method by adding code like this:</p>
 147 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">module_level_parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 148 <span class="gp">&gt;&gt;&gt; </span><span class="n">Element</span> <span class="o">=</span> <span class="n">module_level_parser</span><span class="o">.</span><span class="n">makeelement</span>
 149 </pre></div>
 150 <p>While the <tt class="docutils literal">XML()</tt> and <tt class="docutils literal">HTML()</tt> factories also depend on the default
 151 parser, you can pass them a different parser as second argument:</p>
 152 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">element</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XML</span><span class="p">(</span><span class="s">"&lt;test/&gt;"</span><span class="p">)</span>
 153 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">element</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 154 <span class="go">False</span>
 155
 156 <span class="gp">&gt;&gt;&gt; </span><span class="n">element</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XML</span><span class="p">(</span><span class="s">"&lt;test/&gt;"</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
 157 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">element</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 158 <span class="go">True</span>
 159 </pre></div>
 160 <p>Whenever you create a document with a parser, it will inherit the lookup
 161 scheme and all subsequent element instantiations for this document will use
 162 it:</p>
 163 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">element</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">fromstring</span><span class="p">(</span><span class="s">"&lt;test/&gt;"</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
 164 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">element</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 165 <span class="go">True</span>
 166 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">SubElement</span><span class="p">(</span><span class="n">element</span><span class="p">,</span> <span class="s">"subel"</span><span class="p">)</span>
 167 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">el</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 168 <span class="go">True</span>
 169 </pre></div>
 170 <p>For testing code in the Python interpreter and for small projects, you
 171 may also consider setting a lookup scheme on the default parser.  To
 172 avoid interfering with other modules, however, it is usually a better
 173 idea to use a dedicated parser for each module (or a parser pool when
 174 using threads) and then register the required lookup scheme only for
 175 this parser.</p>
 176 <div class="section" id="default-class-lookup">
 177 <h2>Default class lookup</h2>
 178 <p>This is the most simple lookup mechanism.  It always returns the default
 179 element class.  Consequently, no further fallbacks are supported, but this
 180 scheme is a nice fallback for other custom lookup mechanisms.</p>
 181 <p>Usage:</p>
 182 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementDefaultClassLookup</span><span class="p">()</span>
 183 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 184 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 185 </pre></div>
 186 <p>Note that the default for new parsers is to use the global fallback, which is
 187 also the default lookup (if not configured otherwise).</p>
 188 <p>To change the default element implementation, you can pass your new class to
 189 the constructor.  While it accepts classes for <tt class="docutils literal">element</tt>, <tt class="docutils literal">comment</tt> and
 190 <tt class="docutils literal">pi</tt> nodes, most use cases will only override the element class:</p>
 191 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">makeelement</span><span class="p">(</span><span class="s">"myelement"</span><span class="p">)</span>
 192 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">el</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 193 <span class="go">False</span>
 194
 195 <span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementDefaultClassLookup</span><span class="p">(</span><span class="n">element</span><span class="o">=</span><span class="n">honk</span><span class="p">)</span>
 196 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 197
 198 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">makeelement</span><span class="p">(</span><span class="s">"myelement"</span><span class="p">)</span>
 199 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">isinstance</span><span class="p">(</span><span class="n">el</span><span class="p">,</span> <span class="n">honk</span><span class="p">))</span>
 200 <span class="go">True</span>
 201 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span><span class="o">.</span><span class="n">honking</span>
 202 <span class="go">False</span>
 203 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">makeelement</span><span class="p">(</span><span class="s">"myelement"</span><span class="p">,</span> <span class="n">honking</span><span class="o">=</span><span class="s">'true'</span><span class="p">)</span>
 204 <span class="gp">&gt;&gt;&gt; </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">el</span><span class="p">)</span>
 205 <span class="go">b'&lt;myelement honking="true"/&gt;'</span>
 206 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span><span class="o">.</span><span class="n">honking</span>
 207 <span class="go">True</span>
 208 </pre></div>
 209 </div>
 210 <div class="section" id="namespace-class-lookup">
 211 <h2>Namespace class lookup</h2>
 212 <p>This is an advanced lookup mechanism that supports namespace/tag-name specific
 213 element classes.  You can select it by calling:</p>
 214 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementNamespaceClassLookup</span><span class="p">()</span>
 215 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 216 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 217 </pre></div>
 218 <p>See the separate section on <a class="reference external" href="#implementing-namespaces">implementing namespaces</a> below to learn how to
 219 make use of it.</p>
 220 <p>This scheme supports a fallback mechanism that is used in the case where the
 221 namespace is not found or no class was registered for the element name.
 222 Normally, the default class lookup is used here.  To change it, pass the
 223 desired fallback lookup scheme to the constructor:</p>
 224 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">fallback</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementDefaultClassLookup</span><span class="p">(</span><span class="n">element</span><span class="o">=</span><span class="n">honk</span><span class="p">)</span>
 225 <span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementNamespaceClassLookup</span><span class="p">(</span><span class="n">fallback</span><span class="p">)</span>
 226 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 227 </pre></div>
 228 </div>
 229 <div class="section" id="attribute-based-lookup">
 230 <h2>Attribute based lookup</h2>
 231 <p>This scheme uses a mapping from attribute values to classes.  An attribute
 232 name is set at initialisation time and is then used to find the corresponding
 233 value in a dictionary.  It is set up as follows:</p>
 234 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">id_class_mapping</span> <span class="o">=</span> <span class="p">{</span><span class="s">'1234'</span> <span class="p">:</span> <span class="n">honk</span><span class="p">}</span> <span class="c"># maps attribute values to classes</span>
 235
 236 <span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">AttributeBasedElementClassLookup</span><span class="p">(</span>
 237 <span class="gp">... </span>                                     <span class="s">'id'</span><span class="p">,</span> <span class="n">id_class_mapping</span><span class="p">)</span>
 238 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 239 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 240 </pre></div>
 241 <p>And here is how to use it:</p>
 242 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">xml</span> <span class="o">=</span> <span class="s">'&lt;a id="123"&gt;&lt;b id="1234"/&gt;&lt;b id="1234" honking="true"/&gt;&lt;/a&gt;'</span>
 243 <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">fromstring</span><span class="p">(</span><span class="n">xml</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
 244
 245 <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="o">.</span><span class="n">honking</span>       <span class="c"># id does not match !</span>
 246 <span class="gt">Traceback (most recent call last):</span>
 247 <span class="gr">AttributeError</span>: <span class="n">'lxml.etree._Element' object has no attribute 'honking'</span>
 248
 249 <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">honking</span>
 250 <span class="go">False</span>
 251 <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span><span class="o">.</span><span class="n">honking</span>
 252 <span class="go">True</span>
 253 </pre></div>
 254 <p>This lookup scheme uses its fallback if the attribute is not found or
 255 its value is not in the mapping.  Normally, the default class lookup
 256 is used here.  If you want to use the namespace lookup, for example,
 257 you can use this code:</p>
 258 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">fallback</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementNamespaceClassLookup</span><span class="p">()</span>
 259 <span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">AttributeBasedElementClassLookup</span><span class="p">(</span>
 260 <span class="gp">... </span>                      <span class="s">'id'</span><span class="p">,</span> <span class="n">id_class_mapping</span><span class="p">,</span> <span class="n">fallback</span><span class="p">)</span>
 261 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 262 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 263 </pre></div>
 264 </div>
 265 <div class="section" id="custom-element-class-lookup">
 266 <h2>Custom element class lookup</h2>
 267 <p>This is the most customisable way of finding element classes on a per-element
 268 basis.  It allows you to implement a custom lookup scheme in a subclass:</p>
 269 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">MyLookup</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">CustomElementClassLookup</span><span class="p">):</span>
 270 <span class="gp">... </span>    <span class="k">def</span> <span class="nf">lookup</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node_type</span><span class="p">,</span> <span class="n">document</span><span class="p">,</span> <span class="n">namespace</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
 271 <span class="gp">... </span>        <span class="k">return</span> <span class="n">honk</span> <span class="c"># be a bit more selective here ...</span>
 272
 273 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 274 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">MyLookup</span><span class="p">())</span>
 275 </pre></div>
 276 <p>The <tt class="docutils literal">.lookup()</tt> method must return either None (which triggers the
 277 fallback mechanism) or a subclass of <tt class="docutils literal">lxml.etree.ElementBase</tt>.  It
 278 can take any decision it wants based on the node type (one of
 279 "element", "comment", "PI", "entity"), the XML document of the
 280 element, or its namespace or tag name.</p>
 281 </div>
 282 <div class="section" id="tree-based-element-class-lookup-in-python">
 283 <h2>Tree based element class lookup in Python</h2>
 284 <p>Taking more elaborate decisions than allowed by the custom scheme is
 285 difficult to achieve in pure Python, as it results in a
 286 chicken-and-egg problem.  It would require access to the tree - before
 287 the elements in the tree have been instantiated as Python Element
 288 proxies.</p>
 289 <p>Luckily, there is a way to do this.  The <tt class="docutils literal">PythonElementClassLookup</tt>
 290 works similar to the custom lookup scheme:</p>
 291 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">MyLookup</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">PythonElementClassLookup</span><span class="p">):</span>
 292 <span class="gp">... </span>    <span class="k">def</span> <span class="nf">lookup</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">document</span><span class="p">,</span> <span class="n">element</span><span class="p">):</span>
 293 <span class="gp">... </span>        <span class="k">return</span> <span class="n">MyElementClass</span> <span class="c"># defined elsewhere</span>
 294
 295 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 296 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">MyLookup</span><span class="p">())</span>
 297 </pre></div>
 298 <p>As before, the first argument to the <tt class="docutils literal">lookup()</tt> method is the opaque
 299 document instance that contains the Element.  The second arguments is a
 300 lightweight Element proxy implementation that is only valid during the lookup.
 301 Do not try to keep a reference to it.  Once the lookup is finished, the proxy
 302 will become invalid.  You will get an <tt class="docutils literal">AssertionError</tt> if you access any of
 303 the properties or methods outside the scope of the lookup call where they were
 304 instantiated.</p>
 305 <p>During the lookup, the element object behaves mostly like a normal Element
 306 instance.  It provides the properties <tt class="docutils literal">tag</tt>, <tt class="docutils literal">text</tt>, <tt class="docutils literal">tail</tt> etc. and
 307 supports indexing, slicing and the <tt class="docutils literal">getchildren()</tt>, <tt class="docutils literal">getparent()</tt>
 308 etc. methods.  It does <em>not</em> support iteration, nor does it support any kind
 309 of modification.  All of its properties are read-only and it cannot be removed
 310 or inserted into other trees.  You can use it as a starting point to freely
 311 traverse the tree and collect any kind of information that its elements
 312 provide.  Once you have taken the decision which class to use for this
 313 element, you can simply return it and have lxml take care of cleaning up the
 314 instantiated proxy classes.</p>
 315 <p>Sidenote: this lookup scheme originally lived in a separate module called
 316 <tt class="docutils literal">lxml.pyclasslookup</tt>.</p>
 317 </div>
 318 </div>
 319 <div class="section" id="generating-xml-with-custom-classes">
 320 <h1>Generating XML with custom classes</h1>
 321 <p>Up to lxml 2.1, you could not instantiate proxy classes yourself.
 322 Only lxml.etree could do that when creating an object representation
 323 of an existing XML element.  Since lxml 2.2, however, instantiating
 324 this class will simply create a new Element:</p>
 325 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">honk</span><span class="p">(</span><span class="n">honking</span> <span class="o">=</span> <span class="s">'true'</span><span class="p">)</span>
 326 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span><span class="o">.</span><span class="n">tag</span>
 327 <span class="go">'honk'</span>
 328 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span><span class="o">.</span><span class="n">honking</span>
 329 <span class="go">True</span>
 330 </pre></div>
 331 <p>Note, however, that the proxy you create here will be garbage
 332 collected just like any other proxy.  You can therefore not count on
 333 lxml.etree using the same class that you instantiated when you access
 334 this Element a second time after letting its reference go.  You should
 335 therefore always use a corresponding class lookup scheme that returns
 336 your Element proxy classes for the elements that they create.  The
 337 <tt class="docutils literal">ElementNamespaceClassLookup</tt> is generally a good match.</p>
 338 <p>You can use custom Element classes to quickly create XML fragments:</p>
 339 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">hale</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">ElementBase</span><span class="p">):</span> <span class="k">pass</span>
 340 <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">bopp</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">ElementBase</span><span class="p">):</span> <span class="k">pass</span>
 341
 342 <span class="gp">&gt;&gt;&gt; </span><span class="n">el</span> <span class="o">=</span> <span class="n">hale</span><span class="p">(</span> <span class="s">"some "</span><span class="p">,</span> <span class="n">honk</span><span class="p">(</span><span class="n">honking</span> <span class="o">=</span> <span class="s">'true'</span><span class="p">),</span> <span class="n">bopp</span><span class="p">,</span> <span class="s">" text"</span> <span class="p">)</span>
 343
 344 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">el</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s">'unicode'</span><span class="p">))</span>
 345 <span class="go">&lt;hale&gt;some &lt;honk honking="true"/&gt;&lt;bopp/&gt; text&lt;/hale&gt;</span>
 346 </pre></div>
 347 </div>
 348 <div class="section" id="id1">
 349 <h1>Implementing namespaces</h1>
 350 <p>lxml allows you to implement namespaces, in a rather literal sense.  After
 351 setting up the namespace class lookup mechanism as described above, you can
 352 build a new element namespace (or retrieve an existing one) by calling the
 353 <tt class="docutils literal">get_namespace(uri)</tt> method of the lookup:</p>
 354 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">lookup</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementNamespaceClassLookup</span><span class="p">()</span>
 355 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLParser</span><span class="p">()</span>
 356 <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">set_element_class_lookup</span><span class="p">(</span><span class="n">lookup</span><span class="p">)</span>
 357
 358 <span class="gp">&gt;&gt;&gt; </span><span class="n">namespace</span> <span class="o">=</span> <span class="n">lookup</span><span class="o">.</span><span class="n">get_namespace</span><span class="p">(</span><span class="s">'http://hui.de/honk'</span><span class="p">)</span>
 359 </pre></div>
 360 <p>and then register the new element type with that namespace, say, under the tag
 361 name <tt class="docutils literal">honk</tt>:</p>
 362 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">namespace</span><span class="p">[</span><span class="s">'honk'</span><span class="p">]</span> <span class="o">=</span> <span class="n">honk</span>
 363 </pre></div>
 364 <p>If you have many Element classes declared in one module, and they are
 365 all named like the elements they create, you can simply use
 366 <tt class="docutils literal"><span class="pre">namespace.update(vars())</span></tt> at the end of your module to declare them
 367 automatically.  The implementation is smart enough to ignore
 368 everything that is not an Element class.</p>
 369 <p>After this, you create and use your XML elements through the normal API of
 370 lxml:</p>
 371 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">xml</span> <span class="o">=</span> <span class="s">'&lt;honk xmlns="http://hui.de/honk" honking="true"/&gt;'</span>
 372 <span class="gp">&gt;&gt;&gt; </span><span class="n">honk_element</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XML</span><span class="p">(</span><span class="n">xml</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
 373 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="o">.</span><span class="n">honking</span><span class="p">)</span>
 374 <span class="go">True</span>
 375 </pre></div>
 376 <p>The same works when creating elements by hand:</p>
 377 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">honk_element</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">makeelement</span><span class="p">(</span><span class="s">'{http://hui.de/honk}honk'</span><span class="p">,</span>
 378 <span class="gp">... </span>                                  <span class="n">honking</span><span class="o">=</span><span class="s">'true'</span><span class="p">)</span>
 379 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="o">.</span><span class="n">honking</span><span class="p">)</span>
 380 <span class="go">True</span>
 381 </pre></div>
 382 <p>Essentially, what this allows you to do, is to give Elements a custom API
 383 based on their namespace and tag name.</p>
 384 <p>A somewhat related topic are <a class="reference external" href="extensions.html">extension functions</a> which use a similar
 385 mechanism for registering extension functions in XPath and XSLT.</p>
 386 <p>In the setup example above, we associated the <tt class="docutils literal">honk</tt> Element class
 387 only with the 'honk' element.  If an XML tree contains different
 388 elements in the same namespace, they do not pick up the same
 389 implementation:</p>
 390 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">xml</span> <span class="o">=</span> <span class="s">'&lt;honk xmlns="http://hui.de/honk" honking="true"&gt;&lt;bla/&gt;&lt;/honk&gt;'</span>
 391 <span class="gp">&gt;&gt;&gt; </span><span class="n">honk_element</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XML</span><span class="p">(</span><span class="n">xml</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
 392 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="o">.</span><span class="n">honking</span><span class="p">)</span>
 393 <span class="go">True</span>
 394 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">honking</span><span class="p">)</span>
 395 <span class="gt">Traceback (most recent call last):</span>
 396 <span class="c">...</span>
 397 <span class="gr">AttributeError</span>: <span class="n">'lxml.etree._Element' object has no attribute 'honking'</span>
 398 </pre></div>
 399 <p>You can therefore provide one implementation per element name in each
 400 namespace and have lxml select the right one on the fly.  If you want one
 401 element implementation per namespace (ignoring the element name) or prefer
 402 having a common class for most elements except a few, you can specify a
 403 default implementation for an entire namespace by registering that class with
 404 the empty element name (None).</p>
 405 <p>You may consider following an object oriented approach here.  If you build a
 406 class hierarchy of element classes, you can also implement a base class for a
 407 namespace that is used if no specific element class is provided.  Again, you
 408 can just pass None as an element name:</p>
 409 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">HonkNSElement</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">ElementBase</span><span class="p">):</span>
 410 <span class="gp">... </span>   <span class="k">def</span> <span class="nf">honk</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
 411 <span class="gp">... </span>      <span class="k">return</span> <span class="s">"HONK"</span>
 412 <span class="gp">&gt;&gt;&gt; </span><span class="n">namespace</span><span class="p">[</span><span class="bp">None</span><span class="p">]</span> <span class="o">=</span> <span class="n">HonkNSElement</span> <span class="c"># default Element for namespace</span>
 413
 414 <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">HonkElement</span><span class="p">(</span><span class="n">HonkNSElement</span><span class="p">):</span>
 415 <span class="gp">... </span>   <span class="nd">@property</span>
 416 <span class="gp">... </span>   <span class="k">def</span> <span class="nf">honking</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
 417 <span class="gp">... </span>      <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'honking'</span><span class="p">)</span> <span class="o">==</span> <span class="s">'true'</span>
 418 <span class="gp">&gt;&gt;&gt; </span><span class="n">namespace</span><span class="p">[</span><span class="s">'honk'</span><span class="p">]</span> <span class="o">=</span> <span class="n">HonkElement</span> <span class="c"># Element for specific tag</span>
 419 </pre></div>
 420 <p>Now you can rely on lxml to always return objects of type HonkNSElement or its
 421 subclasses for elements of this namespace:</p>
 422 <div class="syntax"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">xml</span> <span class="o">=</span> <span class="s">'&lt;honk xmlns="http://hui.de/honk" honking="true"&gt;&lt;bla/&gt;&lt;/honk&gt;'</span>
 423 <span class="gp">&gt;&gt;&gt; </span><span class="n">honk_element</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">XML</span><span class="p">(</span><span class="n">xml</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
 424
 425 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">honk_element</span><span class="p">))</span>
 426 <span class="go">&lt;class 'HonkElement'&gt;</span>
 427 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">honk_element</span><span class="p">[</span><span class="mi">0</span><span class="p">]))</span>
 428 <span class="go">&lt;class 'HonkNSElement'&gt;</span>
 429
 430 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="o">.</span><span class="n">honking</span><span class="p">)</span>
 431 <span class="go">True</span>
 432 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="o">.</span><span class="n">honk</span><span class="p">())</span>
 433 <span class="go">HONK</span>
 434
 435 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">honk</span><span class="p">())</span>
 436 <span class="go">HONK</span>
 437 <span class="gp">&gt;&gt;&gt; </span><span class="k">print</span><span class="p">(</span><span class="n">honk_element</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">honking</span><span class="p">)</span>
 438 <span class="gt">Traceback (most recent call last):</span>
 439 <span class="c">...</span>
 440 <span class="gr">AttributeError</span>: <span class="n">'HonkNSElement' object has no attribute 'honking'</span>
 441 </pre></div>
 442 </div>
 443 </div>
 444 <div class="footer">
 445 <hr class="footer" />
 446 Generated on: 2012-07-31.
 447
 448 </div>
 449 </body>
 450 </html>