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class="menu item"><a href="lxml-source-howto.html#where-to-start">Where to start?</a></li><li class="menu item"><a href="lxml-source-howto.html#lxml-etree">lxml.etree</a></li><li class="menu item"><a href="lxml-source-howto.html#python-modules">Python modules</a></li><li class="menu item"><a href="lxml-source-howto.html#lxml-objectify">lxml.objectify</a></li><li class="menu item"><a href="lxml-source-howto.html#lxml-html">lxml.html</a></li></ul></li></ul><ul class="menu foreign" id="changes 4 2 1-menu"><li class="menu title"><a href="changes-4.2.1.html">Release Changelog</a></li></ul><ul class="menu foreign" id="credits-menu"><li class="menu title"><a href="credits.html">Credits</a><ul class="submenu"><li class="menu item"><a href="credits.html#main-contributors">Main contributors</a></li><li class="menu item"><a href="credits.html#special-thanks-goes-to">Special thanks goes to:</a></li></ul></li></ul></li><li><a href="http://lxml.de/sitemap.html">Sitemap</a></li></ul></div></div><h1 class="title">The public C-API of lxml.etree</h1>

<p>As of version 1.1, lxml.etree provides a public C-API.  This allows external
C extensions to efficiently access public functions and classes of lxml,
without going through the Python API.</p>
<p>The API is described in the file <a class="reference external" href="https://github.com/lxml/lxml/blob/master/src/lxml/includes/etreepublic.pxd">etreepublic.pxd</a>, which is directly
c-importable by extension modules implemented in <a class="reference external" href="http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/">Pyrex</a> or <a class="reference external" href="http://cython.org">Cython</a>.</p>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#passing-generated-trees-through-python" id="id1">Passing generated trees through Python</a></li>
<li><a class="reference internal" href="#writing-external-modules-in-cython" id="id2">Writing external modules in Cython</a></li>
<li><a class="reference internal" href="#writing-external-modules-in-c" id="id3">Writing external modules in C</a></li>
</ul>
</div>
<div class="section" id="passing-generated-trees-through-python">
<h1>Passing generated trees through Python</h1>
<p>This is the most simple way to integrate with lxml.  It does not require
any C-level integration but uses a Python function to wrap an externally
generated libxml2 document in lxml.</p>
<p>The external module that creates the libxml2 tree must pack the document
pointer into a <a class="reference external" href="https://docs.python.org/3/c-api/capsule.html">PyCapsule</a>
object.  This can then be passed into lxml with the function
<tt class="docutils literal">lxml.etree.adopt_external_document()</tt>.  It also takes an optional lxml
parser instance to associate with the document, in order to configure the
Element class lookup, relative URL lookups, etc.</p>
<p>See the <a class="reference external" href="api/lxml.etree-module.html#adopt_external_document">API reference</a>
for further details.</p>
<p>The same functionality is available as part of the public C-API in form
of the C function <tt class="docutils literal">adoptExternalDocument()</tt>.</p>
</div>
<div class="section" id="writing-external-modules-in-cython">
<h1>Writing external modules in Cython</h1>
<p>This is the easiest way of extending lxml at the C level.  A <a class="reference external" href="http://cython.org">Cython</a>
(or <a class="reference external" href="http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/">Pyrex</a>) module should start like this:</p>
<pre class="literal-block">
# My Cython extension

# import the public functions and classes of lxml.etree
cimport etreepublic as cetree

# import the lxml.etree module in Python
cdef object etree
from lxml import etree

# initialize the access to the C-API of lxml.etree
cetree.import_lxml__etree()
</pre>
<p>From this line on, you can access all public functions of lxml.etree
from the <tt class="docutils literal">cetree</tt> namespace like this:</p>
<pre class="literal-block">
# build a tag name from namespace and element name
py_tag = cetree.namespacedNameFromNsName("http://some/url", "myelement")
</pre>
<p>Public lxml classes are easily subclassed.  For example, to implement
and set a new default element class, you can write Cython code like
the following:</p>
<pre class="literal-block">
from etreepublic cimport ElementBase
cdef class NewElementClass(ElementBase):
     def set_value(self, myval):
         self.set("my_attribute", myval)

etree.set_element_class_lookup(
     etree.DefaultElementClassLookup(element=NewElementClass))
</pre>
</div>
<div class="section" id="writing-external-modules-in-c">
<h1>Writing external modules in C</h1>
<p>If you really feel like it, you can also interface with lxml.etree straight
from C code.  All you have to do is include the header file for the public
API, import the <tt class="docutils literal">lxml.etree</tt> module and then call the import function:</p>
<div class="syntax"><pre><span class="cm">/* My C extension */</span>

<span class="cm">/* common includes */</span>
<span class="cp">#include</span> <span class="cpf">"Python.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"stdio.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"string.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"stdarg.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/xmlversion.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/encoding.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/hash.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/tree.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/xmlIO.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/xmlsave.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/globals.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"libxml/xmlstring.h"</span><span class="cp"></span>

<span class="cm">/* lxml.etree specific includes */</span>
<span class="cp">#include</span> <span class="cpf">"lxml-version.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"etree_defs.h"</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"etree.h"</span><span class="cp"></span>

<span class="cm">/* setup code */</span>
<span class="n">import_lxml__etree</span><span class="p">()</span>
</pre></div>
<p>Note that including <tt class="docutils literal">etree.h</tt> does not automatically include the
header files it requires.  Note also that the above list of common
includes may not be sufficient.</p>
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