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class="title">APIs specific to lxml.etree</h1>
<p>lxml.etree tries to follow established APIs wherever possible. Sometimes,
however, the need to expose a feature in an easy way led to the invention of a
new API. This page describes the major differences and a few additions to the
main ElementTree API.</p>
<p>For a complete reference of the API, see the <a class="reference external" href="api/index.html">generated API
documentation</a>.</p>
<p>Separate pages describe the support for <a class="reference external" href="parsing.html">parsing XML</a>, executing <a class="reference external" href="xpathxslt.html">XPath and
XSLT</a>, <a class="reference external" href="validation.html">validating XML</a> and interfacing with other XML tools through the
<a class="reference external" href="sax.html">SAX-API</a>.</p>
<p>lxml is extremely extensible through <a class="reference external" href="extensions.html">XPath functions in Python</a>, custom
<a class="reference external" href="element_classes.html">Python element classes</a>, custom <a class="reference external" href="resolvers.html">URL resolvers</a> and even <a class="reference external" href="capi.html">at the C-level</a>.</p>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#lxml-etree" id="id1">lxml.etree</a></li>
<li><a class="reference internal" href="#other-element-apis" id="id2">Other Element APIs</a></li>
<li><a class="reference internal" href="#trees-and-documents" id="id3">Trees and Documents</a></li>
<li><a class="reference internal" href="#iteration" id="id4">Iteration</a></li>
<li><a class="reference internal" href="#error-handling-on-exceptions" id="id5">Error handling on exceptions</a></li>
<li><a class="reference internal" href="#error-logging" id="id6">Error logging</a></li>
<li><a class="reference internal" href="#serialisation" id="id7">Serialisation</a></li>
<li><a class="reference internal" href="#incremental-xml-generation" id="id8">Incremental XML generation</a></li>
<li><a class="reference internal" href="#cdata" id="id9">CDATA</a></li>
<li><a class="reference internal" href="#xinclude-and-elementinclude" id="id10">XInclude and ElementInclude</a></li>
<li><a class="reference internal" href="#write-c14n-on-elementtree" id="id11">write_c14n on ElementTree</a></li>
</ul>
</div>
<div class="section" id="lxml-etree">
<h1>lxml.etree</h1>
<p>lxml.etree tries to follow the <a class="reference external" href="http://effbot.org/zone/element-index.htm">ElementTree API</a> wherever it can. There are
however some incompatibilities (see <a class="reference external" href="compatibility.html">compatibility</a>). The extensions are
documented here.</p>
<p>If you need to know which version of lxml is installed, you can access the
<tt class="docutils literal">lxml.etree.LXML_VERSION</tt> attribute to retrieve a version tuple. Note,
however, that it did not exist before version 1.0, so you will get an
AttributeError in older versions. The versions of libxml2 and libxslt are
available through the attributes <tt class="docutils literal">LIBXML_VERSION</tt> and <tt class="docutils literal">LIBXSLT_VERSION</tt>.</p>
<p>The following examples usually assume this to be executed first:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">lxml</span> <span class="kn">import</span> <span class="n">etree</span>
</pre></div>
</div>
<div class="section" id="other-element-apis">
<h1>Other Element APIs</h1>
<p>While lxml.etree itself uses the ElementTree API, it is possible to replace
the Element implementation by <a class="reference external" href="element_classes.html">custom element subclasses</a>. This has been
used to implement well-known XML APIs on top of lxml. For example, lxml ships
with a data-binding implementation called <a class="reference external" href="objectify.html">objectify</a>, which is similar to
the <a class="reference external" href="http://uche.ogbuji.net/tech/4suite/amara/">Amara bindery</a> tool.</p>
<p>lxml.etree comes with a number of <a class="reference external" href="element_classes.html#setting-up-a-class-lookup-scheme">different lookup schemes</a> to customize the
mapping between libxml2 nodes and the Element classes used by lxml.etree.</p>
</div>
<div class="section" id="trees-and-documents">
<h1>Trees and Documents</h1>
<p>Compared to the original ElementTree API, lxml.etree has an extended tree
model. It knows about parents and siblings of elements:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">root</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="s2">"root"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">a</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">root</span><span class="p">,</span> <span class="s2">"a"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b</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">root</span><span class="p">,</span> <span class="s2">"b"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">c</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">root</span><span class="p">,</span> <span class="s2">"c"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">d</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">root</span><span class="p">,</span> <span class="s2">"d"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">e</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">d</span><span class="p">,</span> <span class="s2">"e"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b</span><span class="o">.</span><span class="n">getparent</span><span class="p">()</span> <span class="o">==</span> <span class="n">root</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">b</span><span class="o">.</span><span class="n">getnext</span><span class="p">()</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">c</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">c</span><span class="o">.</span><span class="n">getprevious</span><span class="p">()</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">b</span>
</pre></div>
<p>Elements always live within a document context in lxml. This implies that
there is also a notion of an absolute document root. You can retrieve an
ElementTree for the root node of a document from any of its elements.</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">d</span><span class="o">.</span><span class="n">getroottree</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">tree</span><span class="o">.</span><span class="n">getroot</span><span class="p">()</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">root</span>
</pre></div>
<p>Note that this is different from wrapping an Element in an ElementTree. You
can use ElementTrees to create XML trees with an explicit root node:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">ElementTree</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">tree</span><span class="o">.</span><span class="n">getroot</span><span class="p">()</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">d</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">tree</span><span class="p">)</span>
<span class="go">b'<d><e/></d>'</span>
</pre></div>
<p>ElementTree objects are serialised as complete documents, including
preceding or trailing processing instructions and comments.</p>
<p>All operations that you run on such an ElementTree (like XPath, XSLT, etc.)
will understand the explicitly chosen root as root node of a document. They
will not see any elements outside the ElementTree. However, ElementTrees do
not modify their Elements:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">element</span> <span class="o">=</span> <span class="n">tree</span><span class="o">.</span><span class="n">getroot</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">element</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">d</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">element</span><span class="o">.</span><span class="n">getparent</span><span class="p">()</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">root</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">element</span><span class="o">.</span><span class="n">getroottree</span><span class="p">()</span><span class="o">.</span><span class="n">getroot</span><span class="p">()</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">root</span>
</pre></div>
<p>The rule is that all operations that are applied to Elements use either the
Element itself as reference point, or the absolute root of the document that
contains this Element (e.g. for absolute XPath expressions). All operations
on an ElementTree use its explicit root node as reference.</p>
</div>
<div class="section" id="iteration">
<h1>Iteration</h1>
<p>The ElementTree API makes Elements iterable to supports iteration over their
children. Using the tree defined above, we get:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="p">[</span> <span class="n">child</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">child</span> <span class="ow">in</span> <span class="n">root</span> <span class="p">]</span>
<span class="go">['a', 'b', 'c', 'd']</span>
</pre></div>
<p>To iterate in the opposite direction, use the builtin <tt class="docutils literal">reversed()</tt> function
that exists in Python 2.4 and later.</p>
<p>Tree traversal should use the <tt class="docutils literal">element.iter()</tt> method:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="p">[</span> <span class="n">el</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">el</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iter</span><span class="p">()</span> <span class="p">]</span>
<span class="go">['root', 'a', 'b', 'c', 'd', 'e']</span>
</pre></div>
<p>lxml.etree also supports this, but additionally features an extended API for
iteration over the children, following/preceding siblings, ancestors and
descendants of an element, as defined by the respective XPath axis:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="p">[</span> <span class="n">child</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">child</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iterchildren</span><span class="p">()</span> <span class="p">]</span>
<span class="go">['a', 'b', 'c', 'd']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">child</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">child</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iterchildren</span><span class="p">(</span><span class="nb">reversed</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span> <span class="p">]</span>
<span class="go">['d', 'c', 'b', 'a']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">sibling</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">sibling</span> <span class="ow">in</span> <span class="n">b</span><span class="o">.</span><span class="n">itersiblings</span><span class="p">()</span> <span class="p">]</span>
<span class="go">['c', 'd']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">sibling</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">sibling</span> <span class="ow">in</span> <span class="n">c</span><span class="o">.</span><span class="n">itersiblings</span><span class="p">(</span><span class="n">preceding</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span> <span class="p">]</span>
<span class="go">['b', 'a']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">ancestor</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">ancestor</span> <span class="ow">in</span> <span class="n">e</span><span class="o">.</span><span class="n">iterancestors</span><span class="p">()</span> <span class="p">]</span>
<span class="go">['d', 'root']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">el</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">el</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iterdescendants</span><span class="p">()</span> <span class="p">]</span>
<span class="go">['a', 'b', 'c', 'd', 'e']</span>
</pre></div>
<p>Note how <tt class="docutils literal">element.iterdescendants()</tt> does not include the element
itself, as opposed to <tt class="docutils literal">element.iter()</tt>. The latter effectively
implements the 'descendant-or-self' axis in XPath.</p>
<p>All of these iterators support one (or more, since lxml 3.0) additional
arguments that filter the generated elements by tag name:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="p">[</span> <span class="n">child</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">child</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iterchildren</span><span class="p">(</span><span class="s1">'a'</span><span class="p">)</span> <span class="p">]</span>
<span class="go">['a']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">child</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">child</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">iterchildren</span><span class="p">(</span><span class="s1">'a'</span><span class="p">)</span> <span class="p">]</span>
<span class="go">[]</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">el</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">el</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iterdescendants</span><span class="p">(</span><span class="s1">'d'</span><span class="p">)</span> <span class="p">]</span>
<span class="go">['d']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">el</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">el</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iter</span><span class="p">(</span><span class="s1">'d'</span><span class="p">)</span> <span class="p">]</span>
<span class="go">['d']</span>
<span class="gp">>>> </span><span class="p">[</span> <span class="n">el</span><span class="o">.</span><span class="n">tag</span> <span class="k">for</span> <span class="n">el</span> <span class="ow">in</span> <span class="n">root</span><span class="o">.</span><span class="n">iter</span><span class="p">(</span><span class="s1">'d'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">)</span> <span class="p">]</span>
<span class="go">['a', 'd']</span>
</pre></div>
<p>Note that the order of the elements is determined by the iteration order,
which is the document order in most cases (except for preceding siblings
and ancestors, where it is the reversed document order). The order of
the tag selection arguments is irrelevant, as you can see in the last
example.</p>
<p>The most common way to traverse an XML tree is depth-first, which
traverses the tree in document order. This is implemented by the
<tt class="docutils literal">.iter()</tt> method. While there is no dedicated method for
breadth-first traversal, it is almost as simple if you use the
<tt class="docutils literal">collections.deque</tt> type that is available in Python 2.4 and later.</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">root</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="s1">'<root><a><b/><c/></a><d><e/></d></root>'</span><span class="p">)</span>
<span class="gp">>>> </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">root</span><span class="p">,</span> <span class="n">pretty_print</span><span class="o">=</span><span class="bp">True</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">'unicode'</span><span class="p">))</span>
<span class="go"><root></span>
<span class="go"> <a></span>
<span class="go"> <b/></span>
<span class="go"> <c/></span>
<span class="go"> </a></span>
<span class="go"> <d></span>
<span class="go"> <e/></span>
<span class="go"> </d></span>
<span class="go"></root></span>
<span class="gp">>>> </span><span class="n">queue</span> <span class="o">=</span> <span class="n">deque</span><span class="p">([</span><span class="n">root</span><span class="p">])</span>
<span class="gp">>>> </span><span class="k">while</span> <span class="n">queue</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">el</span> <span class="o">=</span> <span class="n">queue</span><span class="o">.</span><span class="n">popleft</span><span class="p">()</span> <span class="c1"># pop next element</span>
<span class="gp">... </span> <span class="n">queue</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="n">el</span><span class="p">)</span> <span class="c1"># append its children</span>
<span class="gp">... </span> <span class="k">print</span><span class="p">(</span><span class="n">el</span><span class="o">.</span><span class="n">tag</span><span class="p">)</span>
<span class="go">root</span>
<span class="go">a</span>
<span class="go">d</span>
<span class="go">b</span>
<span class="go">c</span>
<span class="go">e</span>
</pre></div>
<p>See also the section on the utility functions <tt class="docutils literal">iterparse()</tt> and
<tt class="docutils literal">iterwalk()</tt> in the <a class="reference external" href="parsing.html#iterparse-and-iterwalk">parser documentation</a>.</p>
</div>
<div class="section" id="error-handling-on-exceptions">
<h1>Error handling on exceptions</h1>
<p>Libxml2 provides error messages for failures, be it during parsing, XPath
evaluation or schema validation. The preferred way of accessing them is
through the local <tt class="docutils literal">error_log</tt> property of the respective evaluator or
transformer object. See their documentation for details.</p>
<p>However, lxml also keeps a global error log of all errors that occurred at the
application level. Whenever an exception is raised, you can retrieve the
errors that occurred and "might have" lead to the problem from the error log
copy attached to the exception:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">clear_error_log</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">broken_xml</span> <span class="o">=</span> <span class="s1">'''</span>
<span class="gp">... </span><span class="s1"><root></span>
<span class="gp">... </span><span class="s1"> <a></span>
<span class="gp">... </span><span class="s1"></root></span>
<span class="gp">... </span><span class="s1">'''</span>
<span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">etree</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="n">StringIO</span><span class="p">(</span><span class="n">broken_xml</span><span class="p">))</span>
<span class="gp">... </span><span class="k">except</span> <span class="n">etree</span><span class="o">.</span><span class="n">XMLSyntaxError</span><span class="p">,</span> <span class="n">e</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">pass</span> <span class="c1"># just put the exception into e</span>
</pre></div>
<p>Once you have caught this exception, you can access its <tt class="docutils literal">error_log</tt> property
to retrieve the log entries or filter them by a specific type, error domain or
error level:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">log</span> <span class="o">=</span> <span class="n">e</span><span class="o">.</span><span class="n">error_log</span><span class="o">.</span><span class="n">filter_from_level</span><span class="p">(</span><span class="n">etree</span><span class="o">.</span><span class="n">ErrorLevels</span><span class="o">.</span><span class="n">FATAL</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">log</span><span class="p">)</span>
<span class="go"><string>:4:8:FATAL:PARSER:ERR_TAG_NAME_MISMATCH: Opening and ending tag mismatch: a line 3 and root</span>
<span class="go"><string>:5:1:FATAL:PARSER:ERR_TAG_NOT_FINISHED: Premature end of data in tag root line 2</span>
</pre></div>
<p>This might look a little cryptic at first, but it is the information that
libxml2 gives you. At least the message at the end should give you a hint
what went wrong and you can see that the fatal errors (FATAL) happened during
parsing (PARSER) lines 4, column 8 and line 5, column 1 of a string (<string>,
or the filename if available). Here, PARSER is the so-called error domain,
see <tt class="docutils literal">lxml.etree.ErrorDomains</tt> for that. You can get it from a log entry
like this:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">entry</span> <span class="o">=</span> <span class="n">log</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">entry</span><span class="o">.</span><span class="n">domain_name</span><span class="p">)</span>
<span class="go">PARSER</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">entry</span><span class="o">.</span><span class="n">type_name</span><span class="p">)</span>
<span class="go">ERR_TAG_NAME_MISMATCH</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">entry</span><span class="o">.</span><span class="n">filename</span><span class="p">)</span>
<span class="go"><string></span>
</pre></div>
<p>There is also a convenience attribute <tt class="docutils literal">last_error</tt> that returns the last
error or fatal error that occurred:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">entry</span> <span class="o">=</span> <span class="n">e</span><span class="o">.</span><span class="n">error_log</span><span class="o">.</span><span class="n">last_error</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">entry</span><span class="o">.</span><span class="n">domain_name</span><span class="p">)</span>
<span class="go">PARSER</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">entry</span><span class="o">.</span><span class="n">type_name</span><span class="p">)</span>
<span class="go">ERR_TAG_NOT_FINISHED</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">entry</span><span class="o">.</span><span class="n">filename</span><span class="p">)</span>
<span class="go"><string></span>
</pre></div>
</div>
<div class="section" id="error-logging">
<h1>Error logging</h1>
<p>lxml.etree supports logging libxml2 messages to the Python stdlib logging
module. This is done through the <tt class="docutils literal">etree.PyErrorLog</tt> class. It disables the
error reporting from exceptions and forwards log messages to a Python logger.
To use it, see the descriptions of the function <tt class="docutils literal">etree.useGlobalPythonLog</tt>
and the class <tt class="docutils literal">etree.PyErrorLog</tt> for help. Note that this does not affect
the local error logs of XSLT, XMLSchema, etc.</p>
</div>
<div class="section" id="serialisation">
<h1>Serialisation</h1>
<p>lxml.etree has direct support for pretty printing XML output. Functions like
<tt class="docutils literal">ElementTree.write()</tt> and <tt class="docutils literal">tostring()</tt> support it through a keyword
argument:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">root</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="s2">"<root><test/></root>"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">root</span><span class="p">)</span>
<span class="go">b'<root><test/></root>'</span>
<span class="gp">>>> </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">root</span><span class="p">,</span> <span class="n">pretty_print</span><span class="o">=</span><span class="bp">True</span><span class="p">))</span>
<span class="go"><root></span>
<span class="go"> <test/></span>
<span class="go"></root></span>
</pre></div>
<p>Note the newline that is appended at the end when pretty printing the
output. It was added in lxml 2.0.</p>
<p>By default, lxml (just as ElementTree) outputs the XML declaration only if it
is required by the standard:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">unicode_root</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="s2">u"t</span><span class="se">\u3120</span><span class="s2">st"</span> <span class="p">)</span>
<span class="gp">>>> </span><span class="n">unicode_root</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="s2">u"t</span><span class="se">\u0A0A</span><span class="s2">st"</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">unicode_root</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s2">"utf-8"</span><span class="p">)</span>
<span class="go">b'<t\xe3\x84\xa0st>t\xe0\xa8\x8ast</t\xe3\x84\xa0st>'</span>
<span class="gp">>>> </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">unicode_root</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s2">"iso-8859-1"</span><span class="p">))</span>
<span class="go"><?xml version='1.0' encoding='iso-8859-1'?></span>
<span class="go"><t&#12576;st>t&#2570;st</t&#12576;st></span>
</pre></div>
<p>Also see the general remarks on <a class="reference external" href="parsing.html#python-unicode-strings">Unicode support</a>.</p>
<p>You can enable or disable the declaration explicitly by passing another
keyword argument for the serialisation:</p>
<div class="syntax"><pre><span class="gp">>>> </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">root</span><span class="p">,</span> <span class="n">xml_declaration</span><span class="o">=</span><span class="bp">True</span><span class="p">))</span>
<span class="go"><?xml version='1.0' encoding='ASCII'?></span>
<span class="go"><root><test/></root></span>
<span class="gp">>>> </span><span class="n">unicode_root</span><span class="o">.</span><span class="n">clear</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">unicode_root</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s2">"UTF-16LE"</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">xml_declaration</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
<span class="go">b'<\x00t\x00 1s\x00t\x00/\x00>\x00'</span>
</pre></div>
<p>Note that a standard compliant XML parser will not consider the last line
well-formed XML if the encoding is not explicitly provided somehow, e.g. in an
underlying transport protocol:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">notxml</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">unicode_root</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s2">"UTF-16LE"</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">xml_declaration</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">root</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">notxml</span><span class="p">)</span> <span class="c1">#doctest: +ELLIPSIS</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">lxml.etree.XMLSyntaxError</span>: <span class="n">...</span>
</pre></div>
<p>Since version 2.3, the serialisation can override the internal subset
of the document with a user provided DOCTYPE:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">xml</span> <span class="o">=</span> <span class="s1">'<!DOCTYPE root></span><span class="se">\n</span><span class="s1"><root/>'</span>
<span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="n">StringIO</span><span class="p">(</span><span class="n">xml</span><span class="p">))</span>
<span class="gp">>>> </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">tree</span><span class="p">))</span>
<span class="go"><!DOCTYPE root></span>
<span class="go"><root/></span>
<span class="gp">>>> </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">tree</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">doctype</span><span class="o">=</span><span class="s1">'<!DOCTYPE root SYSTEM "/tmp/test.dtd">'</span><span class="p">))</span>
<span class="go"><!DOCTYPE root SYSTEM "/tmp/test.dtd"></span>
<span class="go"><root/></span>
</pre></div>
<p>The content will be encoded, but otherwise copied verbatim into the
output stream. It is therefore left to the user to take care for a
correct doctype format, including the name of the root node.</p>
</div>
<div class="section" id="incremental-xml-generation">
<h1>Incremental XML generation</h1>
<p>Since version 3.1, lxml provides an <tt class="docutils literal">xmlfile</tt> API for incrementally
generating XML using the <tt class="docutils literal">with</tt> statement. It's main purpose is to
freely and safely mix surrounding elements with pre-built in-memory
trees, e.g. to write out large documents that consist mostly of
repetitive subtrees (like database dumps). But it can be useful in
many cases where memory consumption matters or where XML is naturally
generated in sequential steps. Since lxml 3.4.1, there is an equivalent
context manager for HTML serialisation called <tt class="docutils literal">htmlfile</tt>.</p>
<p>The API can serialise to real files (given as file path or file
object), as well as file-like objects, e.g. <tt class="docutils literal">io.BytesIO()</tt>.
Here is a simple example:</p>
<pre class="literal-block">
>>> f = BytesIO()
>>> with etree.xmlfile(f) as xf:
... with xf.element('abc'):
... xf.write('text')
>>> print(f.getvalue().decode('utf-8'))
<abc>text</abc>
</pre>
<p><tt class="docutils literal">xmlfile()</tt> accepts a file path as first argument, or a file(-like)
object, as in the example above. In the first case, it takes care to
open and close the file itself, whereas file(-like) objects are not
closed by default. This is left to the code that opened them. Since
lxml 3.4, however, you can pass the argument <tt class="docutils literal">close=True</tt> to make
lxml call the object's <tt class="docutils literal">.close()</tt> method when exiting the xmlfile
context manager.</p>
<p>To insert pre-constructed Elements and subtrees, just pass them
into <tt class="docutils literal">write()</tt>:</p>
<pre class="literal-block">
>>> f = BytesIO()
>>> with etree.xmlfile(f) as xf:
... with xf.element('abc'):
... with xf.element('in'):
...
... for value in '123':
... # construct a really complex XML tree
... el = etree.Element('xyz', attr=value)
...
... xf.write(el)
...
... # no longer needed, discard it right away!
... el = None
>>> print(f.getvalue().decode('utf-8'))
<abc><in><xyz attr="1"/><xyz attr="2"/><xyz attr="3"/></in></abc>
</pre>
<p>It is a common pattern to have one or more nested <tt class="docutils literal">element()</tt>
blocks, and then build in-memory XML subtrees in a loop (using the
ElementTree API, the builder API, XSLT, or whatever) and write them
out into the XML file one after the other. That way, they can be
removed from memory right after their construction, which can largely
reduce the memory footprint of an application, while keeping the
overall XML generation easy, safe and correct.</p>
<p>Together with Python coroutines, this can be used to generate XML
in an asynchronous, non-blocking fashion, e.g. for a stream protocol
like the instant messaging protocol
<a class="reference external" href="https://en.wikipedia.org/wiki/Extensible_Messaging_and_Presence_Protocol">XMPP</a>:</p>
<pre class="literal-block">
def writer(out_stream):
with xmlfile(out_stream) as xf:
with xf.element('{http://etherx.jabber.org/streams}stream'):
while True:
el = (yield)
xf.write(el)
xf.flush()
w = writer(stream)
next(w) # start writing (run up to 'yield')
</pre>
<p>Then, whenever XML elements are available for writing, call</p>
<pre class="literal-block">
w.send(element)
</pre>
<p>And when done:</p>
<pre class="literal-block">
w.close()
</pre>
<p>Note the additional <tt class="docutils literal">xf.flush()</tt> call in the example above, which is
available since lxml 3.4. Normally, the output stream is buffered to
avoid excessive I/O calls. Whenever the internal buffer fills up, its
content is written out. In the case above, however, we want to make
sure that each message that we write (i.e. each element subtree) is
written out immediately, so we flush the content explicitly at the
right point.</p>
<p>Alternatively, if buffering is not desired at all, it can be disabled
by passing the flag <tt class="docutils literal">buffered=False</tt> into <tt class="docutils literal">xmlfile()</tt> (also since
lxml 3.4).</p>
<p>Here is a similar example using an async coroutine in Py3.5 or later, which is
supported since lxml 4.0. The output stream is expected to have methods
<tt class="docutils literal">async def write(self, data)</tt> and <tt class="docutils literal">async def close(self)</tt> in this case.</p>
<pre class="literal-block">
async def writer(out_stream, xml_messages):
async with xmlfile(out_stream) as xf:
async with xf.element('{http://etherx.jabber.org/streams}stream'):
async for el in xml_messages:
await xf.write(el)
await xf.flush()
class DummyAsyncOut(object):
async def write(self, data):
print(data.decode('utf8'))
async def close(self):
pass
stream = DummyAsyncOut()
async_writer = writer(stream, async_message_stream)
</pre>
</div>
<div class="section" id="cdata">
<h1>CDATA</h1>
<p>By default, lxml's parser will strip CDATA sections from the tree and
replace them by their plain text content. As real applications for
CDATA are rare, this is the best way to deal with this issue.</p>
<p>However, in some cases, keeping CDATA sections or creating them in a
document is required to adhere to existing XML language definitions.
For these special cases, you can instruct the parser to leave CDATA
sections in the document:</p>
<div class="syntax"><pre><span class="gp">>>> </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><span class="n">strip_cdata</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">root</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="s1">'<root><![CDATA[test]]></root>'</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">text</span>
<span class="go">'test'</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">root</span><span class="p">)</span>
<span class="go">b'<root><![CDATA[test]]></root>'</span>
</pre></div>
<p>Note how the <tt class="docutils literal">.text</tt> property does not give any indication that the
text content is wrapped by a CDATA section. If you want to make sure
your data is wrapped by a CDATA block, you can use the <tt class="docutils literal">CDATA()</tt>
text wrapper:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="s1">'test'</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">text</span>
<span class="go">'test'</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">root</span><span class="p">)</span>
<span class="go">b'<root>test</root>'</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">CDATA</span><span class="p">(</span><span class="n">root</span><span class="o">.</span><span class="n">text</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">text</span>
<span class="go">'test'</span>
<span class="gp">>>> </span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">root</span><span class="p">)</span>
<span class="go">b'<root><![CDATA[test]]></root>'</span>
</pre></div>
</div>
<div class="section" id="xinclude-and-elementinclude">
<h1>XInclude and ElementInclude</h1>
<p>You can let lxml process xinclude statements in a document by calling the
xinclude() method on a tree:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="n">StringIO</span><span class="p">(</span><span class="s1">'''</span><span class="se">\</span>
<span class="gp">... </span><span class="s1"><doc xmlns:xi="http://www.w3.org/2001/XInclude"></span>
<span class="gp">... </span><span class="s1"><foo/></span>
<span class="gp">... </span><span class="s1"><xi:include href="doc/test.xml" /></span>
<span class="gp">... </span><span class="s1"></doc>'''</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">tree</span><span class="o">.</span><span class="n">xinclude</span><span class="p">()</span>
<span class="gp">>>> </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">tree</span><span class="o">.</span><span class="n">getroot</span><span class="p">()))</span>
<span class="go"><doc xmlns:xi="http://www.w3.org/2001/XInclude"></span>
<span class="go"><foo/></span>
<span class="go"><a xml:base="doc/test.xml"/></span>
<span class="go"></doc></span>
</pre></div>
<p>Note that the ElementTree compatible <a class="reference external" href="http://effbot.org/zone/element-xinclude.htm">ElementInclude</a> module is also supported
as <tt class="docutils literal">lxml.ElementInclude</tt>. It has the additional advantage of supporting
custom <a class="reference external" href="resolvers.html">URL resolvers</a> at the Python level. The normal XInclude mechanism
cannot deploy these. If you need ElementTree compatibility or custom
resolvers, you have to stick to the external Python module.</p>
</div>
<div class="section" id="write-c14n-on-elementtree">
<h1>write_c14n on ElementTree</h1>
<p>The lxml.etree.ElementTree class has a method write_c14n, which takes a file
object as argument. This file object will receive an UTF-8 representation of
the canonicalized form of the XML, following the W3C C14N recommendation. For
example:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="n">StringIO</span><span class="p">(</span><span class="s1">'<a><b/></a>'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">etree</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">f2</span> <span class="o">=</span> <span class="n">StringIO</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">tree</span><span class="o">.</span><span class="n">write_c14n</span><span class="p">(</span><span class="n">f2</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">f2</span><span class="o">.</span><span class="n">getvalue</span><span class="p">()</span><span class="o">.</span><span class="n">decode</span><span class="p">(</span><span class="s2">"utf-8"</span><span class="p">))</span>
<span class="go"><a><b></b></a></span>
</pre></div>
</div>
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