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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 3-menu"><li class="menu title"><a href="changes-4.2.3.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="/sitemap.html">Sitemap</a></li></ul></div></div><h1 class="title">lxml FAQ - Frequently Asked Questions</h1>
<p>Frequently asked questions on lxml. See also the notes on <a class="reference external" href="compatibility.html">compatibility</a> to
<a class="reference external" href="http://effbot.org/zone/element-index.htm">ElementTree</a>.</p>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#general-questions" id="id2">General Questions</a><ul>
<li><a class="reference internal" href="#is-there-a-tutorial" id="id3">Is there a tutorial?</a></li>
<li><a class="reference internal" href="#where-can-i-find-more-documentation-about-lxml" id="id4">Where can I find more documentation about lxml?</a></li>
<li><a class="reference internal" href="#what-standards-does-lxml-implement" id="id5">What standards does lxml implement?</a></li>
<li><a class="reference internal" href="#who-uses-lxml" id="id6">Who uses lxml?</a></li>
<li><a class="reference internal" href="#what-is-the-difference-between-lxml-etree-and-lxml-objectify" id="id7">What is the difference between lxml.etree and lxml.objectify?</a></li>
<li><a class="reference internal" href="#how-can-i-make-my-application-run-faster" id="id8">How can I make my application run faster?</a></li>
<li><a class="reference internal" href="#what-about-that-trailing-text-on-serialised-elements" id="id9">What about that trailing text on serialised Elements?</a></li>
<li><a class="reference internal" href="#how-can-i-find-out-if-an-element-is-a-comment-or-pi" id="id10">How can I find out if an Element is a comment or PI?</a></li>
<li><a class="reference internal" href="#how-can-i-map-an-xml-tree-into-a-dict-of-dicts" id="id11">How can I map an XML tree into a dict of dicts?</a></li>
<li><a class="reference internal" href="#why-does-lxml-sometimes-return-str-values-for-text-in-python-2" id="id12">Why does lxml sometimes return 'str' values for text in Python 2?</a></li>
<li><a class="reference internal" href="#why-do-i-get-xinclude-or-dtd-lookup-failures-on-some-systems-but-not-on-others" id="id13">Why do I get XInclude or DTD lookup failures on some systems but not on others?</a></li>
<li><a class="reference internal" href="#how-do-namespaces-work-in-lxml" id="id14">How do namespaces work in lxml?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#installation" id="id15">Installation</a><ul>
<li><a class="reference internal" href="#which-version-of-libxml2-and-libxslt-should-i-use-or-require" id="id16">Which version of libxml2 and libxslt should I use or require?</a></li>
<li><a class="reference internal" href="#where-are-the-binary-builds" id="id17">Where are the binary builds?</a></li>
<li><a class="reference internal" href="#why-do-i-get-errors-about-missing-ucs4-symbols-when-installing-lxml" id="id18">Why do I get errors about missing UCS4 symbols when installing lxml?</a></li>
<li><a class="reference internal" href="#my-c-compiler-crashes-on-installation" id="id19">My C compiler crashes on installation</a></li>
</ul>
</li>
<li><a class="reference internal" href="#contributing" id="id20">Contributing</a><ul>
<li><a class="reference internal" href="#why-is-lxml-not-written-in-python" id="id21">Why is lxml not written in Python?</a></li>
<li><a class="reference internal" href="#how-can-i-contribute" id="id22">How can I contribute?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#bugs" id="id23">Bugs</a><ul>
<li><a class="reference internal" href="#my-application-crashes" id="id24">My application crashes!</a></li>
<li><a class="reference internal" href="#my-application-crashes-on-macos-x" id="id25">My application crashes on MacOS-X!</a></li>
<li><a class="reference internal" href="#i-think-i-have-found-a-bug-in-lxml-what-should-i-do" id="id26">I think I have found a bug in lxml. What should I do?</a></li>
<li><a class="reference internal" href="#how-do-i-know-a-bug-is-really-in-lxml-and-not-in-libxml2" id="id27">How do I know a bug is really in lxml and not in libxml2?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#id1" id="id28">Threading</a><ul>
<li><a class="reference internal" href="#can-i-use-threads-to-concurrently-access-the-lxml-api" id="id29">Can I use threads to concurrently access the lxml API?</a></li>
<li><a class="reference internal" href="#does-my-program-run-faster-if-i-use-threads" id="id30">Does my program run faster if I use threads?</a></li>
<li><a class="reference internal" href="#would-my-single-threaded-program-run-faster-if-i-turned-off-threading" id="id31">Would my single-threaded program run faster if I turned off threading?</a></li>
<li><a class="reference internal" href="#why-can-t-i-reuse-xslt-stylesheets-in-other-threads" id="id32">Why can't I reuse XSLT stylesheets in other threads?</a></li>
<li><a class="reference internal" href="#my-program-crashes-when-run-with-mod-python-pyro-zope-plone" id="id33">My program crashes when run with mod_python/Pyro/Zope/Plone/...</a></li>
</ul>
</li>
<li><a class="reference internal" href="#parsing-and-serialisation" id="id34">Parsing and Serialisation</a><ul>
<li><a class="reference internal" href="#why-doesn-t-the-pretty-print-option-reformat-my-xml-output" id="id35">Why doesn't the <tt class="docutils literal">pretty_print</tt> option reformat my XML output?</a></li>
<li><a class="reference internal" href="#why-can-t-lxml-parse-my-xml-from-unicode-strings" id="id36">Why can't lxml parse my XML from unicode strings?</a></li>
<li><a class="reference internal" href="#can-lxml-parse-from-file-objects-opened-in-unicode-text-mode" id="id37">Can lxml parse from file objects opened in unicode/text mode?</a></li>
<li><a class="reference internal" href="#what-is-the-difference-between-str-xslt-doc-and-xslt-doc-write" id="id38">What is the difference between str(xslt(doc)) and xslt(doc).write() ?</a></li>
<li><a class="reference internal" href="#why-can-t-i-just-delete-parents-or-clear-the-root-node-in-iterparse" id="id39">Why can't I just delete parents or clear the root node in iterparse()?</a></li>
<li><a class="reference internal" href="#how-do-i-output-null-characters-in-xml-text" id="id40">How do I output null characters in XML text?</a></li>
<li><a class="reference internal" href="#is-lxml-vulnerable-to-xml-bombs" id="id41">Is lxml vulnerable to XML bombs?</a></li>
<li><a class="reference internal" href="#how-do-i-use-lxml-safely-as-a-web-service-endpoint" id="id42">How do I use lxml safely as a web-service endpoint?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#xpath-and-document-traversal" id="id43">XPath and Document Traversal</a><ul>
<li><a class="reference internal" href="#what-are-the-findall-and-xpath-methods-on-element-tree" id="id44">What are the <tt class="docutils literal">findall()</tt> and <tt class="docutils literal">xpath()</tt> methods on Element(Tree)?</a></li>
<li><a class="reference internal" href="#why-doesn-t-findall-support-full-xpath-expressions" id="id45">Why doesn't <tt class="docutils literal">findall()</tt> support full XPath expressions?</a></li>
<li><a class="reference internal" href="#how-can-i-find-out-which-namespace-prefixes-are-used-in-a-document" id="id46">How can I find out which namespace prefixes are used in a document?</a></li>
<li><a class="reference internal" href="#how-can-i-specify-a-default-namespace-for-xpath-expressions" id="id47">How can I specify a default namespace for XPath expressions?</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="general-questions">
<h1>General Questions</h1>
<div class="section" id="is-there-a-tutorial">
<h2>Is there a tutorial?</h2>
<p>Read the <a class="reference external" href="tutorial.html">lxml.etree Tutorial</a>. While this is still work in progress
(just as any good documentation), it provides an overview of the most
important concepts in <tt class="docutils literal">lxml.etree</tt>. If you want to help out,
improving the tutorial is a very good place to start.</p>
<p>There is also a <a class="reference external" href="http://effbot.org/zone/element.htm">tutorial for ElementTree</a> which works for
<tt class="docutils literal">lxml.etree</tt>. The documentation of the <a class="reference external" href="api.html">extended etree API</a> also
contains many examples for <tt class="docutils literal">lxml.etree</tt>. Fredrik Lundh's <a class="reference external" href="http://effbot.org/zone/element-lib.htm">element
library</a> contains a lot of nice recipes that show how to solve common
tasks in ElementTree and lxml.etree. To learn using
<tt class="docutils literal">lxml.objectify</tt>, read the <a class="reference external" href="objectify.html">objectify documentation</a>.</p>
<p>John Shipman has written another tutorial called <a class="reference external" href="http://www.nmt.edu/tcc/help/pubs/pylxml/">Python XML
processing with lxml</a> that contains lots of examples. Liza Daly
wrote a nice article about high-performance aspects when <a class="reference external" href="http://www.ibm.com/developerworks/xml/library/x-hiperfparse/">parsing
large files with lxml</a>.</p>
</div>
<div class="section" id="where-can-i-find-more-documentation-about-lxml">
<h2>Where can I find more documentation about lxml?</h2>
<p>There is a lot of documentation on the web and also in the Python
standard library documentation, as lxml implements the well-known
<a class="reference external" href="http://effbot.org/zone/element-index.htm">ElementTree API</a> and tries to follow its documentation as closely as
possible. The recipes in Fredrik Lundh's <a class="reference external" href="http://effbot.org/zone/element-lib.htm">element library</a> are
generally worth taking a look at. There are a couple of issues where
lxml cannot keep up compatibility. They are described in the
<a class="reference external" href="compatibility.html">compatibility</a> documentation.</p>
<p>The lxml specific extensions to the API are described by individual
files in the <tt class="docutils literal">doc</tt> directory of the source distribution and on <a class="reference external" href="http://lxml.de/#documentation">the
web page</a>.</p>
<p>The <a class="reference external" href="api/index.html">generated API documentation</a> is a comprehensive API reference
for the lxml package.</p>
</div>
<div class="section" id="what-standards-does-lxml-implement">
<h2>What standards does lxml implement?</h2>
<p>The compliance to XML Standards depends on the support in libxml2 and libxslt.
Here is a quote from <a class="reference external" href="http://xmlsoft.org/">http://xmlsoft.org/</a>:</p>
<blockquote>
In most cases libxml2 tries to implement the specifications in a relatively
strictly compliant way. As of release 2.4.16, libxml2 passed all 1800+ tests
from the OASIS XML Tests Suite.</blockquote>
<p>lxml currently supports libxml2 2.6.20 or later, which has even better
support for various XML standards. The important ones are:</p>
<ul class="simple">
<li>XML 1.0</li>
<li>HTML 4</li>
<li>XML namespaces</li>
<li>XML Schema 1.0</li>
<li>XPath 1.0</li>
<li>XInclude 1.0</li>
<li>XSLT 1.0</li>
<li>EXSLT</li>
<li>XML catalogs</li>
<li>canonical XML</li>
<li>RelaxNG</li>
<li>xml:id</li>
<li>xml:base</li>
</ul>
<p>Support for XML Schema is currently not 100% complete in libxml2, but
is definitely very close to compliance. Schematron is supported in
two ways, the best being the original ISO Schematron reference
implementation via XSLT. libxml2 also supports loading documents
through HTTP and FTP.</p>
<p>For <a class="reference external" href="http://relaxng.org/compact-tutorial-20030326.html">RelaxNG Compact Syntax</a>
support, there is a tool called <a class="reference external" href="http://www.gnosis.cx/download/relax/">rnc2rng</a>,
written by David Mertz, which you might be able to use from Python. Failing
that, <a class="reference external" href="http://code.google.com/p/jing-trang/">trang</a> is the 'official'
command line tool (written in Java) to do the conversion.</p>
</div>
<div class="section" id="who-uses-lxml">
<h2>Who uses lxml?</h2>
<p>As an XML library, lxml is often used under the hood of in-house
server applications, such as web servers or applications that
facilitate some kind of content management. Many people who deploy
<a class="reference external" href="http://www.zope.org/">Zope</a>, <a class="reference external" href="http://www.plone.org/">Plone</a> or <a class="reference external" href="https://www.djangoproject.com/">Django</a> use it together with lxml in the background,
without speaking publicly about it. Therefore, it is hard to get an
idea of who uses it, and the following list of 'users and projects we
know of' is very far from a complete list of lxml's users.</p>
<p>Also note that the compatibility to the ElementTree library does not
require projects to set a hard dependency on lxml - as long as they do
not take advantage of lxml's enhanced feature set.</p>
<ul class="simple">
<li><a class="reference external" href="http://code.google.com/p/cssutils/source/browse/trunk/examples/style.py?r=917">cssutils</a>,
a CSS parser and toolkit, can be used with <tt class="docutils literal">lxml.cssselect</tt></li>
<li><a class="reference external" href="http://www.openplans.org/projects/deliverance/project-home">Deliverance</a>,
a content theming tool</li>
<li><a class="reference external" href="http://www.enfoldsystems.com/Products/Proxy/4">Enfold Proxy 4</a>,
a web server accelerator with on-the-fly XSLT processing</li>
<li><a class="reference external" href="http://lists.wald.intevation.org/pipermail/inteproxy-devel/2007-February/000000.html">Inteproxy</a>,
a secure HTTP proxy</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/lwebstring">lwebstring</a>,
an XML template engine</li>
<li><a class="reference external" href="https://openpyxl.readthedocs.io/">openpyxl</a>,
a library to read/write MS Excel 2007 files</li>
<li><a class="reference external" href="http://permalink.gmane.org/gmane.comp.python.lxml.devel/3250">OpenXMLlib</a>,
a library for handling OpenXML document meta data</li>
<li><a class="reference external" href="http://www.psychopy.org/">PsychoPy</a>,
psychology software in Python</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/pycoon">Pycoon</a>,
a WSGI web development framework based on XML pipelines</li>
<li><a class="reference external" href="http://pycsw.org">pycsw</a>,
an <a class="reference external" href="http://opengeospatial.org/standards/cat">OGC CSW</a> server implementation written in Python</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/pyquery">PyQuery</a>,
a query framework for XML/HTML, similar to jQuery for JavaScript</li>
<li><a class="reference external" href="http://github.com/mikemaccana/python-docx">python-docx</a>,
a package for handling Microsoft's Word OpenXML format</li>
<li><a class="reference external" href="http://beta.rambler.ru/srch?query=python+lxml&searchtype=web">Rambler</a>,
a meta search engine that aggregates different data sources</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/rdfadict">rdfadict</a>,
an RDFa parser with a simple dictionary-like interface.</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/xupdate-processor">xupdate-processor</a>,
an XUpdate implementation for lxml.etree</li>
<li><a class="reference external" href="http://docs.diazo.org/">Diazo</a>,
an XSLT-under-the-hood web site theming engine</li>
</ul>
<p>Zope3 and some of its extensions have good support for lxml:</p>
<ul class="simple">
<li><a class="reference external" href="http://pypi.python.org/pypi/gocept.lxml">gocept.lxml</a>,
Zope3 interface bindings for lxml</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/z3c.rml">z3c.rml</a>,
an implementation of ReportLab's RML format</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/zif.sedna">zif.sedna</a>,
an XQuery based interface to the Sedna OpenSource XML database</li>
</ul>
<p>And don't miss the quotes by our generally <a class="reference external" href="http://thread.gmane.org/gmane.comp.python.lxml.devel/3244/focus=3244">happy</a> <a class="reference external" href="http://article.gmane.org/gmane.comp.python.lxml.devel/3246">users</a>, and other
<a class="reference external" href="http://www.google.com/search?as_lq=http:%2F%2Flxml.de%2F">sites that link to lxml</a>. As <a class="reference external" href="http://www.ibm.com/developerworks/xml/library/x-hiperfparse/">Liza Daly</a> puts it: "Many software
products come with the pick-two caveat, meaning that you must choose
only two: speed, flexibility, or readability. When used carefully,
lxml can provide all three."</p>
</div>
<div class="section" id="what-is-the-difference-between-lxml-etree-and-lxml-objectify">
<h2>What is the difference between lxml.etree and lxml.objectify?</h2>
<p>The two modules provide different ways of handling XML. However, objectify
builds on top of lxml.etree and therefore inherits most of its capabilities
and a large portion of its API.</p>
<ul>
<li><p class="first">lxml.etree is a generic API for XML and HTML handling. It aims for
ElementTree <a class="reference external" href="compatibility.html">compatibility</a> and supports the entire XML infoset. It is well
suited for both mixed content and data centric XML. Its generality makes it
the best choice for most applications.</p>
</li>
<li><p class="first">lxml.objectify is a specialized API for XML data handling in a Python object
syntax. It provides a very natural way to deal with data fields stored in a
structurally well defined XML format. Data is automatically converted to
Python data types and can be manipulated with normal Python operators. Look
at the examples in the <a class="reference external" href="objectify.html">objectify documentation</a> to see what it feels like
to use it.</p>
<p>Objectify is not well suited for mixed contents or HTML documents. As it is
built on top of lxml.etree, however, it inherits the normal support for
XPath, XSLT or validation.</p>
</li>
</ul>
</div>
<div class="section" id="how-can-i-make-my-application-run-faster">
<h2>How can I make my application run faster?</h2>
<p>lxml.etree is a very fast library for processing XML. There are, however, <a class="reference external" href="performance.html#the-elementtree-api">a
few caveats</a> involved in the mapping of the powerful libxml2 library to the
simple and convenient ElementTree API. Not all operations are as fast as the
simplicity of the API might suggest, while some use cases can heavily benefit
from finding the right way of doing them. The <a class="reference external" href="performance.html">benchmark page</a> has a
comparison to other ElementTree implementations and a number of tips for
performance tweaking. As with any Python application, the rule of thumb is:
the more of your processing runs in C, the faster your application gets. See
also the section on <a class="reference external" href="#threading">threading</a>.</p>
</div>
<div class="section" id="what-about-that-trailing-text-on-serialised-elements">
<h2>What about that trailing text on serialised Elements?</h2>
<p>The ElementTree tree model defines an Element as a container with a tag name,
contained text, child Elements and a tail text. This means that whenever you
serialise an Element, you will get all parts of that Element:</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><tag>text<child/></tag>tail</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="mi">0</span><span class="p">]))</span>
<span class="go"><tag>text<child/></tag>tail</span>
</pre></div>
<p>Here is an example that shows why not serialising the tail would be
even more surprising from an object point of view:</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">"test"</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="o">=</span> <span class="s2">"TEXT"</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="go"><test>TEXT</test></span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">tail</span> <span class="o">=</span> <span class="s2">"TAIL"</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="go"><test>TEXT</test>TAIL</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">tail</span> <span class="o">=</span> <span class="bp">None</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="go"><test>TEXT</test></span>
</pre></div>
<p>Just imagine a Python list where you append an item and it doesn't
show up when you look at the list.</p>
<p>The <tt class="docutils literal">.tail</tt> property is a huge simplification for the tree model as
it avoids text nodes to appear in the list of children and makes
access to them quick and simple. So this is a benefit in most
applications and simplifies many, many XML tree algorithms.</p>
<p>However, in document-like XML (and especially HTML), the above result can be
unexpected to new users and can sometimes require a bit more overhead. A good
way to deal with this is to use helper functions that copy the Element without
its tail. The <tt class="docutils literal">lxml.html</tt> package also deals with this in a couple of
places, as most HTML algorithms benefit from a tail-free behaviour.</p>
</div>
<div class="section" id="how-can-i-find-out-if-an-element-is-a-comment-or-pi">
<h2>How can I find out if an Element is a comment or PI?</h2>
<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">"<?my PI?><root><!-- empty --></root>"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">tag</span>
<span class="go">'root'</span>
<span class="gp">>>> </span><span class="n">root</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="ow">is</span> <span class="n">etree</span><span class="o">.</span><span class="n">PI</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">root</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">tag</span> <span class="ow">is</span> <span class="n">etree</span><span class="o">.</span><span class="n">Comment</span>
<span class="go">True</span>
</pre></div>
</div>
<div class="section" id="how-can-i-map-an-xml-tree-into-a-dict-of-dicts">
<h2>How can I map an XML tree into a dict of dicts?</h2>
<p>I'm glad you asked.</p>
<div class="syntax"><pre><span class="k">def</span> <span class="nf">recursive_dict</span><span class="p">(</span><span class="n">element</span><span class="p">):</span>
<span class="k">return</span> <span class="n">element</span><span class="o">.</span><span class="n">tag</span><span class="p">,</span> \
<span class="nb">dict</span><span class="p">(</span><span class="nb">map</span><span class="p">(</span><span class="n">recursive_dict</span><span class="p">,</span> <span class="n">element</span><span class="p">))</span> <span class="ow">or</span> <span class="n">element</span><span class="o">.</span><span class="n">text</span>
</pre></div>
<p>Note that this beautiful quick-and-dirty converter expects children
to have unique tag names and will silently overwrite any data that
was contained in preceding siblings with the same name. For any
real-world application of xml-to-dict conversion, you would better
write your own, longer version of this.</p>
</div>
<div class="section" id="why-does-lxml-sometimes-return-str-values-for-text-in-python-2">
<h2>Why does lxml sometimes return 'str' values for text in Python 2?</h2>
<p>In Python 2, lxml's API returns byte strings for plain ASCII text
values, be it for tag names or text in Element content. This is the
same behaviour as known from ElementTree. The reasoning is that ASCII
encoded byte strings are compatible with Unicode strings in Python 2,
but consume less memory (usually by a factor of 2 or 4) and are faster
to create because they do not require decoding. Plain ASCII string
values are very common in XML, so this optimisation is generally worth
it.</p>
<p>In Python 3, lxml always returns Unicode strings for text and names,
as does ElementTree. Since Python 3.3, Unicode strings containing
only characters that can be encoded in ASCII or Latin-1 are generally
as efficient as byte strings. In older versions of Python 3, the
above mentioned drawbacks apply.</p>
</div>
<div class="section" id="why-do-i-get-xinclude-or-dtd-lookup-failures-on-some-systems-but-not-on-others">
<h2>Why do I get XInclude or DTD lookup failures on some systems but not on others?</h2>
<p>To avoid network access, external resources are first looked up in
<a class="reference external" href="https://www.oasis-open.org/committees/entity/spec.html">XML catalogues</a>.
Many systems have them installed by default, but some don't.
On Linux systems, the default place to look is the index file
<tt class="docutils literal">/etc/xml/catalog</tt>, which most importantly provides a mapping from
doctype IDs to locally installed DTD files.</p>
<p>See the <a class="reference external" href="http://xmlsoft.org/catalog.html">libxml2 catalogue documentation</a>
for further information.</p>
</div>
<div class="section" id="how-do-namespaces-work-in-lxml">
<h2>How do namespaces work in lxml?</h2>
<p>The same as in ElementTree. See the <a class="reference external" href="tutorial.html#namespaces">tutorial</a>.</p>
</div>
</div>
<div class="section" id="installation">
<h1>Installation</h1>
<div class="section" id="which-version-of-libxml2-and-libxslt-should-i-use-or-require">
<h2>Which version of libxml2 and libxslt should I use or require?</h2>
<p>It really depends on your application, but the rule of thumb is: more recent
versions contain less bugs and provide more features.</p>
<ul class="simple">
<li>Do not use libxml2 2.6.27 if you want to use XPath (including XSLT). You
will get crashes when XPath errors occur during the evaluation (e.g. for
unknown functions). This happens inside the evaluation call to libxml2, so
there is nothing that lxml can do about it.</li>
<li>Try to use versions of both libraries that were released together. At least
the libxml2 version should not be older than the libxslt version.</li>
<li>If you use XML Schema or Schematron which are still under development, the
most recent version of libxml2 is usually a good bet.</li>
<li>The same applies to XPath, where a substantial number of bugs and memory
leaks were fixed over time. If you encounter crashes or memory leaks in
XPath applications, try a more recent version of libxml2.</li>
<li>For parsing and fixing broken HTML, lxml requires at least libxml2 2.6.21.</li>
<li>For the normal tree handling, however, any libxml2 version starting with
2.6.20 should do.</li>
</ul>
<p>Read the <a class="reference external" href="http://xmlsoft.org/news.html">release notes of libxml2</a> and the <a class="reference external" href="http://xmlsoft.org/XSLT/news.html">release notes of libxslt</a> to
see when (or if) a specific bug has been fixed.</p>
</div>
<div class="section" id="where-are-the-binary-builds">
<h2>Where are the binary builds?</h2>
<p>Thanks to the help by Joar Wandborg, we try to make "<a class="reference external" href="https://www.python.org/dev/peps/pep-0513">manylinux</a>" binary
builds for Linux available shortly after each source release, as they
are very frequently used by continuous integration and/or build servers.</p>
<p>Thanks to the help by Maximilian Hils and the Appveyor build service,
we also try to serve the frequent requests for binary builds available
for Microsoft Windows in a timely fashion, since users of that platform
usually fail to build lxml themselves. Two of the major design issues
of this operating system make this non-trivial for its users: the lack
of a pre-installed standard compiler and the missing package management.</p>
<p>Besides that, Christoph Gohlke generously provides <a class="reference external" href="http://www.lfd.uci.edu/~gohlke/pythonlibs/#lxml">unofficial lxml binary
builds for Windows</a>
that are usually very up to date. Consider using them if you prefer a
binary build over a signed official source release.</p>
</div>
<div class="section" id="why-do-i-get-errors-about-missing-ucs4-symbols-when-installing-lxml">
<h2>Why do I get errors about missing UCS4 symbols when installing lxml?</h2>
<p>You are using a Python installation that was configured for a different
internal Unicode representation than the lxml package you are trying to
install. CPython versions before 3.3 allowed to switch between two types
at build time: the 32 bit encoding UCS4 and the 16 bit encoding UCS2.
Sadly, both are not compatible, so eggs and other binary distributions
can only support the one they were compiled with.</p>
<p>This means that you have to compile lxml from sources for your system. Note
that you do not need Cython for this, the lxml source distribution is directly
compilable on both platform types. See the <a class="reference external" href="build.html">build instructions</a> on how to do
this.</p>
</div>
<div class="section" id="my-c-compiler-crashes-on-installation">
<h2>My C compiler crashes on installation</h2>
<p>lxml consists of a relatively large amount of (Cython) generated C code
in a single source module. Compiling this module requires a lot of free
memory, usually more than half a GB, which can pose problems especially on
shared/cloud build systems.</p>
<p>If your C compiler crashes while building lxml from sources, consider
using one of the binary wheels that we provide. The "<a class="reference external" href="https://www.python.org/dev/peps/pep-0513">manylinux</a>" binaries
should generally work well on most build systems and install substantially
faster than a source build.</p>
</div>
</div>
<div class="section" id="contributing">
<h1>Contributing</h1>
<div class="section" id="why-is-lxml-not-written-in-python">
<h2>Why is lxml not written in Python?</h2>
<p>It <em>almost</em> is.</p>
<p>lxml is not written in plain Python, because it interfaces with two C
libraries: libxml2 and libxslt. Accessing them at the C-level is
required for performance reasons.</p>
<p>However, to avoid writing plain C-code and caring too much about the
details of built-in types and reference counting, lxml is written in
<a class="reference external" href="http://cython.org/">Cython</a>, a superset of the Python language that translates to C-code.
Chances are that if you know Python, you can write <a class="reference external" href="http://docs.cython.org/docs/tutorial.html">code that Cython
accepts</a>. Again, the C-ish style used in the lxml code is just for
performance optimisations. If you want to contribute, don't bother
with the details, a Python implementation of your contribution is
better than none. And keep in mind that lxml's flexible API often
favours an implementation of features in pure Python, without
bothering with C-code at all. For example, the <tt class="docutils literal">lxml.html</tt> package
is written entirely in Python.</p>
<p>Please contact the <a class="reference external" href="http://lxml.de/mailinglist/">mailing list</a> if you need any help.</p>
</div>
<div class="section" id="how-can-i-contribute">
<h2>How can I contribute?</h2>
<p>If you find something that you would like lxml to do (or do better),
then please tell us about it on the <a class="reference external" href="http://lxml.de/mailinglist/">mailing list</a>. Pull requests
on github are always appreciated, especially when accompanied by unit
tests and documentation (doctests would be great). See the <tt class="docutils literal">tests</tt>
subdirectories in the lxml source tree (below the <tt class="docutils literal">src</tt> directory)
and the <a class="reference external" href="http://docutils.sourceforge.net/rst.html">ReST</a> <a class="reference external" href="https://github.com/lxml/lxml/tree/master/doc">text files</a> in the <tt class="docutils literal">doc</tt> directory.</p>
<p>We also have a <a class="reference external" href="https://github.com/lxml/lxml/blob/master/IDEAS.txt">list of missing features</a> that we would like to
implement but didn't due to lack if time. If <em>you</em> find the time,
patches are very welcome.</p>
<p>Besides enhancing the code, there are a lot of places where you can help the
project and its user base. You can</p>
<ul class="simple">
<li>spread the word and write about lxml. Many users (especially new Python
users) have not yet heard about lxml, although our user base is constantly
growing. If you write your own blog and feel like saying something about
lxml, go ahead and do so. If we think your contribution or criticism is
valuable to other users, we may even put a link or a quote on the project
page.</li>
<li>provide code examples for the general usage of lxml or specific problems
solved with lxml. Readable code is a very good way of showing how a library
can be used and what great things you can do with it. Again, if we hear
about it, we can set a link on the project page.</li>
<li>work on the documentation. The web page is generated from a set of <a class="reference external" href="http://docutils.sourceforge.net/rst.html">ReST</a>
<a class="reference external" href="https://github.com/lxml/lxml/tree/master/doc">text files</a>. It is meant both as a representative project page for lxml
and as a site for documenting lxml's API and usage. If you have questions
or an idea how to make it more readable and accessible while you are reading
it, please send a comment to the <a class="reference external" href="http://lxml.de/mailinglist/">mailing list</a>.</li>
<li>enhance the web site. We put some work into making the web site
usable, understandable and also easy to find, but there's always
things that can be done better. You may notice that we are not
top-ranked when searching the web for "Python and XML", so maybe you
have an idea how to improve that.</li>
<li>help with the tutorial. A tutorial is the most important starting point for
new users, so it is important for us to provide an easy to understand guide
into lxml. As all documentation, the tutorial is work in progress, so we
appreciate every helping hand.</li>
<li>improve the docstrings. lxml uses docstrings to support Python's integrated
online <tt class="docutils literal">help()</tt> function. However, sometimes these are not sufficient to
grasp the details of the function in question. If you find such a place,
you can try to write up a better description and send it to the <a class="reference external" href="http://lxml.de/mailinglist/">mailing
list</a>.</li>
</ul>
</div>
</div>
<div class="section" id="bugs">
<h1>Bugs</h1>
<div class="section" id="my-application-crashes">
<h2>My application crashes!</h2>
<p>One of the goals of lxml is "no segfaults", so if there is no clear
warning in the documentation that you were doing something potentially
harmful, you have found a bug and we would like to hear about it.
Please report this bug to the <a class="reference external" href="http://lxml.de/mailinglist/">mailing list</a>. See the section on bug
reporting to learn how to do that.</p>
<p>If your application (or e.g. your web container) uses threads, please
see the FAQ section on <a class="reference external" href="#threading">threading</a> to check if you touch on one of the
potential pitfalls.</p>
<p>In any case, try to reproduce the problem with the latest versions of
libxml2 and libxslt. From time to time, bugs and race conditions are found
in these libraries, so a more recent version might already contain a fix for
your problem.</p>
<p>Remember: even if you see lxml appear in a crash stack trace, it is
not necessarily lxml that <em>caused</em> the crash.</p>
</div>
<div class="section" id="my-application-crashes-on-macos-x">
<h2>My application crashes on MacOS-X!</h2>
<p>This was a common problem up to lxml 2.1.x. Since lxml 2.2, the only
officially supported way to use it on this platform is through a
static build against freshly downloaded versions of libxml2 and
libxslt. See the build instructions for <a class="reference external" href="build.html#building-lxml-on-macos-x">MacOS-X</a>.</p>
</div>
<div class="section" id="i-think-i-have-found-a-bug-in-lxml-what-should-i-do">
<h2>I think I have found a bug in lxml. What should I do?</h2>
<p>First, you should look at the <a class="reference external" href="https://github.com/lxml/lxml/blob/master/CHANGES.txt">current developer changelog</a> to see if this
is a known problem that has already been fixed in the master branch since the
release you are using.</p>
<p>Also, the 'crash' section above has a few good advices what to try to see if
the problem is really in lxml - and not in your setup. Believe it or not,
that happens more often than you might think, especially when old libraries
or even multiple library versions are installed.</p>
<p>You should always try to reproduce the problem with the latest
versions of libxml2 and libxslt - and make sure they are used.
<tt class="docutils literal">lxml.etree</tt> can tell you what it runs with:</p>
<div class="syntax"><pre><span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">from</span> <span class="nn">lxml</span> <span class="kn">import</span> <span class="n">etree</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%-20s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="s1">'Python'</span><span class="p">,</span> <span class="n">sys</span><span class="o">.</span><span class="n">version_info</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%-20s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="s1">'lxml.etree'</span><span class="p">,</span> <span class="n">etree</span><span class="o">.</span><span class="n">LXML_VERSION</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%-20s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="s1">'libxml used'</span><span class="p">,</span> <span class="n">etree</span><span class="o">.</span><span class="n">LIBXML_VERSION</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%-20s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="s1">'libxml compiled'</span><span class="p">,</span> <span class="n">etree</span><span class="o">.</span><span class="n">LIBXML_COMPILED_VERSION</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%-20s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="s1">'libxslt used'</span><span class="p">,</span> <span class="n">etree</span><span class="o">.</span><span class="n">LIBXSLT_VERSION</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%-20s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="s1">'libxslt compiled'</span><span class="p">,</span> <span class="n">etree</span><span class="o">.</span><span class="n">LIBXSLT_COMPILED_VERSION</span><span class="p">))</span>
</pre></div>
<p>If you can figure that the problem is not in lxml but in the
underlying libxml2 or libxslt, you can ask right on the respective
mailing lists, which may considerably reduce the time to find a fix or
work-around. See the next question for some hints on how to do that.</p>
<p>Otherwise, we would really like to hear about it. Please report it to
the <a class="reference external" href="https://bugs.launchpad.net/lxml/">bug tracker</a> or to the <a class="reference external" href="http://lxml.de/mailinglist/">mailing list</a> so that we can fix it.
It is very helpful in this case if you can come up with a short code
snippet that demonstrates your problem. If others can reproduce and
see the problem, it is much easier for them to fix it - and maybe even
easier for you to describe it and get people convinced that it really
is a problem to fix.</p>
<p>It is important that you always report the version of lxml, libxml2
and libxslt that you get from the code snippet above. If we do not
know the library versions you are using, we will ask back, so it will
take longer for you to get a helpful answer.</p>
<p>Since as a user of lxml you are likely a programmer, you might find
<a class="reference external" href="http://www.chiark.greenend.org.uk/~sgtatham/bugs.html">this article on bug reports</a> an interesting read.</p>
</div>
<div class="section" id="how-do-i-know-a-bug-is-really-in-lxml-and-not-in-libxml2">
<h2>How do I know a bug is really in lxml and not in libxml2?</h2>
<p>A large part of lxml's functionality is implemented by libxml2 and
libxslt, so problems that you encounter may be in one or the other.
Knowing the right place to ask will reduce the time it takes to fix
the problem, or to find a work-around.</p>
<p>Both libxml2 and libxslt come with their own command line frontends,
namely <tt class="docutils literal">xmllint</tt> and <tt class="docutils literal">xsltproc</tt>. If you encounter problems with
XSLT processing for specific stylesheets or with validation for
specific schemas, try to run the XSLT with <tt class="docutils literal">xsltproc</tt> or the
validation with <tt class="docutils literal">xmllint</tt> respectively to find out if it fails there
as well. If it does, please report directly to the mailing lists of
the respective project, namely:</p>
<ul class="simple">
<li><a class="reference external" href="http://mail.gnome.org/mailman/listinfo/xml">libxml2 mailing list</a></li>
<li><a class="reference external" href="http://mail.gnome.org/mailman/listinfo/xslt">libxslt mailing list</a></li>
</ul>
<p>On the other hand, everything that seems to be related to Python code,
including custom resolvers, custom XPath functions, etc. is likely
outside of the scope of libxml2/libxslt. If you encounter problems
here or you are not sure where there the problem may come from, please
ask on the lxml mailing list first.</p>
<p>In any case, a good explanation of the problem including some simple
test code and some input data will help us (or the libxml2 developers)
see and understand the problem, which largely increases your chance of
getting help. See the question above for a few hints on what is
helpful here.</p>
</div>
</div>
<div class="section" id="id1">
<h1>Threading</h1>
<div class="section" id="can-i-use-threads-to-concurrently-access-the-lxml-api">
<h2>Can I use threads to concurrently access the lxml API?</h2>
<p>Short answer: yes, if you use lxml 2.2 and later.</p>
<p>Since version 1.1, lxml frees the GIL (Python's global interpreter
lock) internally when parsing from disk and memory, as long as you use
either the default parser (which is replicated for each thread) or
create a parser for each thread yourself. lxml also allows
concurrency during validation (RelaxNG and XMLSchema) and XSL
transformation. You can share RelaxNG, XMLSchema and XSLT objects
between threads.</p>
<p>While you can also share parsers between threads, this will serialize
the access to each of them, so it is better to <tt class="docutils literal">.copy()</tt> parsers or
to just use the default parser if you do not need any special
configuration. The same applies to the XPath evaluators, which use an
internal lock to protect their prepared evaluation contexts. It is
therefore best to use separate evaluator instances in threads.</p>
<p>Warning: Before lxml 2.2, and especially before 2.1, there were
various issues when moving subtrees between different threads, or when
applying XSLT objects from one thread to trees parsed or modified in
another. If you need code to run with older versions, you should
generally avoid modifying trees in other threads than the one it was
generated in. Although this should work in many cases, there are
certain scenarios where the termination of a thread that parsed a tree
can crash the application if subtrees of this tree were moved to other
documents. You should be on the safe side when passing trees between
threads if you either</p>
<ul class="simple">
<li>do not modify these trees and do not move their elements to other
trees, or</li>
<li>do not terminate threads while the trees they parsed are still in
use (e.g. by using a fixed size thread-pool or long-running threads
in processing chains)</li>
</ul>
<p>Since lxml 2.2, even multi-thread pipelines are supported. However,
note that it is more efficient to do all tree work inside one thread,
than to let multiple threads work on a tree one after the other. This
is because trees inherit state from the thread that created them,
which must be maintained when the tree is modified inside another
thread.</p>
</div>
<div class="section" id="does-my-program-run-faster-if-i-use-threads">
<h2>Does my program run faster if I use threads?</h2>
<p>Depends. The best way to answer this is timing and profiling.</p>
<p>The global interpreter lock (GIL) in Python serializes access to the
interpreter, so if the majority of your processing is done in Python
code (walking trees, modifying elements, etc.), your gain will be
close to zero. The more of your XML processing moves into lxml,
however, the higher your gain. If your application is bound by XML
parsing and serialisation, or by very selective XPath expressions and
complex XSLTs, your speedup on multi-processor machines can be
substantial.</p>
<p>See the question above to learn which operations free the GIL to support
multi-threading.</p>
</div>
<div class="section" id="would-my-single-threaded-program-run-faster-if-i-turned-off-threading">
<h2>Would my single-threaded program run faster if I turned off threading?</h2>
<p>Possibly, yes. You can see for yourself by compiling lxml entirely
without threading support. Pass the <tt class="docutils literal"><span class="pre">--without-threading</span></tt> option to
setup.py when building lxml from source. You can also build libxml2
without pthread support (<tt class="docutils literal"><span class="pre">--without-pthreads</span></tt> option), which may add
another bit of performance. Note that this will leave internal data
structures entirely without thread protection, so make sure you really
do not use lxml outside of the main application thread in this case.</p>
</div>
<div class="section" id="why-can-t-i-reuse-xslt-stylesheets-in-other-threads">
<h2>Why can't I reuse XSLT stylesheets in other threads?</h2>
<p>Since later lxml 2.0 versions, you can do this. There is some
overhead involved as the result document needs an additional cleanup
traversal when the input document and/or the stylesheet were created
in other threads. However, on a multi-processor machine, the gain of
freeing the GIL easily covers this drawback.</p>
<p>If you need even the last bit of performance, consider keeping (a copy
of) the stylesheet in thread-local storage, and try creating the input
document(s) in the same thread. And do not forget to benchmark your
code to see if the increased code complexity is really worth it.</p>
</div>
<div class="section" id="my-program-crashes-when-run-with-mod-python-pyro-zope-plone">
<h2>My program crashes when run with mod_python/Pyro/Zope/Plone/...</h2>
<p>These environments can use threads in a way that may not make it obvious when
threads are created and what happens in which thread. This makes it hard to
ensure lxml's threading support is used in a reliable way. Sadly, if problems
arise, they are as diverse as the applications, so it is difficult to provide
any generally applicable solution. Also, these environments are so complex
that problems become hard to debug and even harder to reproduce in a
predictable way. If you encounter crashes in one of these systems, but your
code runs perfectly when started by hand, the following gives you a few hints
for possible approaches to solve your specific problem:</p>
<ul>
<li><p class="first">make sure you use recent versions of libxml2, libxslt and lxml. The
libxml2 developers keep fixing bugs in each release, and lxml also
tries to become more robust against possible pitfalls. So newer
versions might already fix your problem in a reliable way. Version
2.2 of lxml contains many improvements.</p>
</li>
<li><p class="first">make sure the library versions you installed are really used. Do
not rely on what your operating system tells you! Print the version
constants in <tt class="docutils literal">lxml.etree</tt> from within your runtime environment to
make sure it is the case. This is especially a problem under
MacOS-X when newer library versions were installed in addition to
the outdated system libraries. Please read the bugs section
regarding MacOS-X in this FAQ.</p>
</li>
<li><p class="first">if you use <tt class="docutils literal">mod_python</tt>, try setting this option:</p>
<blockquote>
<p>PythonInterpreter main_interpreter</p>
</blockquote>
<p>There was a discussion on the mailing list about this problem:</p>
<blockquote>
<p><a class="reference external" href="http://comments.gmane.org/gmane.comp.python.lxml.devel/2942">http://comments.gmane.org/gmane.comp.python.lxml.devel/2942</a></p>
</blockquote>
</li>
<li><p class="first">in a threaded environment, try to initially import <tt class="docutils literal">lxml.etree</tt>
from the main application thread instead of doing first-time imports
separately in each spawned worker thread. If you cannot control the
thread spawning of your web/application server, an import of
<tt class="docutils literal">lxml.etree</tt> in sitecustomize.py or usercustomize.py may still do
the trick.</p>
</li>
<li><p class="first">compile lxml without threading support by running <tt class="docutils literal">setup.py</tt> with the
<tt class="docutils literal"><span class="pre">--without-threading</span></tt> option. While this might be slower in certain
scenarios on multi-processor systems, it <em>might</em> also keep your application
from crashing, which should be worth more to you than peek performance.
Remember that lxml is fast anyway, so concurrency may not even be worth it.</p>
</li>
<li><p class="first">look out for fancy XSLT stuff like foreign document access or
passing in subtrees trough XSLT variables. This might or might not
work, depending on your specific usage. Again, later versions of
lxml and libxslt provide safer support here.</p>
</li>
<li><p class="first">try copying trees at suspicious places in your code and working with
those instead of a tree shared between threads. Note that the
copying must happen inside the target thread to be effective, not in
the thread that created the tree. Serialising in one thread and
parsing in another is also a simple (and fast) way of separating
thread contexts.</p>
</li>
<li><p class="first">try keeping thread-local copies of XSLT stylesheets, i.e. one per thread,
instead of sharing one. Also see the question above.</p>
</li>
<li><p class="first">you can try to serialise suspicious parts of your code with explicit thread
locks, thus disabling the concurrency of the runtime system.</p>
</li>
<li><p class="first">report back on the mailing list to see if there are other ways to work
around your specific problems. Do not forget to report the version numbers
of lxml, libxml2 and libxslt you are using (see the question on reporting
a bug).</p>
</li>
</ul>
<p>Note that most of these options will degrade performance and/or your
code quality. If you are unsure what to do, please ask on the mailing
list.</p>
</div>
</div>
<div class="section" id="parsing-and-serialisation">
<h1>Parsing and Serialisation</h1>
<div class="section" id="why-doesn-t-the-pretty-print-option-reformat-my-xml-output">
<h2>Why doesn't the <tt class="docutils literal">pretty_print</tt> option reformat my XML output?</h2>
<p>Pretty printing (or formatting) an XML document means adding white space to
the content. These modifications are harmless if they only impact elements in
the document that do not carry (text) data. They corrupt your data if they
impact elements that contain data. If lxml cannot distinguish between
whitespace and data, it will not alter your data. Whitespace is therefore
only added between nodes that do not contain data. This is always the case
for trees constructed element-by-element, so no problems should be expected
here. For parsed trees, a good way to assure that no conflicting whitespace
is left in the tree is the <tt class="docutils literal">remove_blank_text</tt> option:</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">remove_blank_text</span><span class="o">=</span><span class="bp">True</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">filename</span><span class="p">,</span> <span class="n">parser</span><span class="p">)</span>
</pre></div>
<p>This will allow the parser to drop blank text nodes when constructing the
tree. If you now call a serialization function to pretty print this tree,
lxml can add fresh whitespace to the XML tree to indent it.</p>
<p>Note that the <tt class="docutils literal">remove_blank_text</tt> option also uses a heuristic if it
has no definite knowledge about the document's ignorable whitespace.
It will keep blank text nodes that appear after non-blank text nodes
at the same level. This is to prevent document-style XML from losing
content.</p>
<p>The HTMLParser has this structural knowledge built-in, which means that
most whitespace that appears between tags in HTML documents will <em>not</em>
be removed by this option, except in places where it is truly ignorable,
e.g. in the page header, between table structure tags, etc. Therefore,
it is also safe to use this option with the HTMLParser, as it will keep
content like the following intact (i.e. it will not remove the space
that separates the two words):</p>
<div class="syntax"><pre><span class="p"><</span><span class="nt">p</span><span class="p">><</span><span class="nt">b</span><span class="p">></span>some<span class="p"></</span><span class="nt">b</span><span class="p">></span> <span class="p"><</span><span class="nt">em</span><span class="p">></span>text<span class="p"></</span><span class="nt">em</span><span class="p">></</span><span class="nt">p</span><span class="p">></span>
</pre></div>
<p>If you want to be sure all blank text is removed from an XML document
(or just more blank text than the parser does by itself), you have to
use either a DTD to tell the parser which whitespace it can safely
ignore, or remove the ignorable whitespace manually after parsing,
e.g. by setting all tail text to None:</p>
<div class="syntax"><pre><span class="k">for</span> <span class="n">element</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="n">element</span><span class="o">.</span><span class="n">tail</span> <span class="o">=</span> <span class="bp">None</span>
</pre></div>
<p>Fredrik Lundh also has a Python-level function for indenting XML by
appending whitespace to tags. It can be found on his <a class="reference external" href="http://effbot.org/zone/element-lib.htm">element
library</a> recipe page.</p>
</div>
<div class="section" id="why-can-t-lxml-parse-my-xml-from-unicode-strings">
<h2>Why can't lxml parse my XML from unicode strings?</h2>
<p>First of all, XML is explicitly defined as a stream of bytes. It's not
Unicode text. Take a look at the <a class="reference external" href="http://www.w3.org/TR/REC-xml/">XML specification</a>, it's all about byte
sequences and how to map them to text and structure. That leads to rule
number one: do not decode your XML data yourself. That's a part of the
work of an XML parser, and it does it very well. Just pass it your data as
a plain byte stream, it will always do the right thing, by specification.</p>
<p>This also includes not opening XML files in text mode. Make sure you always
use binary mode, or, even better, pass the file path into lxml's <tt class="docutils literal">parse()</tt>
function to let it do the file opening, reading and closing itself. This
is the most simple and most efficient way to do it.</p>
<p>That being said, lxml can read Python unicode strings and even tries to
support them if libxml2 does not. This is because there is one valid use
case for parsing XML from text strings: literal XML fragments in source
code.</p>
<p>However, if the unicode string declares an XML encoding internally
(<tt class="docutils literal"><span class="pre"><?xml</span> <span class="pre">encoding="..."?></span></tt>), parsing is bound to fail, as this encoding is
almost certainly not the real encoding used in Python unicode. The same is
true for HTML unicode strings that contain charset meta tags, although the
problems may be more subtle here. The libxml2 HTML parser may not be able
to parse the meta tags in broken HTML and may end up ignoring them, so even
if parsing succeeds, later handling may still fail with character encoding
errors. Therefore, parsing HTML from unicode strings is a much saner thing
to do than parsing XML from unicode strings.</p>
<p>Note that Python uses different encodings for unicode on different platforms,
so even specifying the real internal unicode encoding is not portable between
Python interpreters. Don't do it.</p>
<p>Python unicode strings with XML data that carry encoding information are
broken. lxml will not parse them. You must provide parsable data in a
valid encoding.</p>
</div>
<div class="section" id="can-lxml-parse-from-file-objects-opened-in-unicode-text-mode">
<h2>Can lxml parse from file objects opened in unicode/text mode?</h2>
<p>Technically, yes. However, you likely do not want to do that, because
it is extremely inefficient. The text encoding that libxml2 uses
internally is UTF-8, so parsing from a Unicode file means that Python
first reads a chunk of data from the file, then decodes it into a new
buffer, and then copies it into a new unicode string object, just to
let libxml2 make yet another copy while encoding it down into UTF-8
in order to parse it. It's clear that this involves a lot more
recoding and copying than when parsing straight from the bytes that
the file contains.</p>
<p>If you really know the encoding better than the parser (e.g. when
parsing HTML that lacks a content declaration), then instead of passing
an encoding parameter into the file object when opening it, create a
new instance of an XMLParser or HTMLParser and pass the encoding into
its constructor. Afterwards, use that parser for parsing, e.g. by
passing it into the <tt class="docutils literal">etree.parse(file, parser)</tt> function. Remember
to open the file in binary mode (mode="rb"), or, if possible, prefer
passing the file path directly into <tt class="docutils literal">parse()</tt> instead of an opened
Python file object.</p>
</div>
<div class="section" id="what-is-the-difference-between-str-xslt-doc-and-xslt-doc-write">
<h2>What is the difference between str(xslt(doc)) and xslt(doc).write() ?</h2>
<p>The str() implementation of the XSLTResultTree class (a subclass of the
ElementTree class) knows about the output method chosen in the stylesheet
(xsl:output), write() doesn't. If you call write(), the result will be a
normal XML tree serialization in the requested encoding. Calling this method
may also fail for XSLT results that are not XML trees (e.g. string results).</p>
<p>If you call str(), it will return the serialized result as specified by the
XSL transform. This correctly serializes string results to encoded Python
strings and honours <tt class="docutils literal">xsl:output</tt> options like <tt class="docutils literal">indent</tt>. This almost
certainly does what you want, so you should only use <tt class="docutils literal">write()</tt> if you are
sure that the XSLT result is an XML tree and you want to override the encoding
and indentation options requested by the stylesheet.</p>
</div>
<div class="section" id="why-can-t-i-just-delete-parents-or-clear-the-root-node-in-iterparse">
<h2>Why can't I just delete parents or clear the root node in iterparse()?</h2>
<p>The <tt class="docutils literal">iterparse()</tt> implementation is based on the libxml2 parser. It
requires the tree to be intact to finish parsing. If you delete or modify
parents of the current node, chances are you modify the structure in a way
that breaks the parser. Normally, this will result in a segfault. Please
refer to the <a class="reference external" href="parsing.html#iterparse-and-iterwalk">iterparse section</a> of the lxml API documentation to find out
what you can do and what you can't do.</p>
</div>
<div class="section" id="how-do-i-output-null-characters-in-xml-text">
<h2>How do I output null characters in XML text?</h2>
<p>Don't. What you would produce is not well-formed XML. XML parsers
will refuse to parse a document that contains null characters. The
right way to embed binary data in XML is using a text encoding such as
uuencode or base64.</p>
</div>
<div class="section" id="is-lxml-vulnerable-to-xml-bombs">
<h2>Is lxml vulnerable to XML bombs?</h2>
<p>This has nothing to do with lxml itself, only with the parser of
libxml2. Since libxml2 version 2.7, the parser imposes hard security
limits on input documents to prevent DoS attacks with forged input
data. Since lxml 2.2.1, you can disable these limits with the
<tt class="docutils literal">huge_tree</tt> parser option if you need to parse <em>really</em> large,
trusted documents. All lxml versions will leave these restrictions
enabled by default.</p>
<p>Note that libxml2 versions of the 2.6 series do not restrict their
parser and are therefore vulnerable to DoS attacks.</p>
<p>Note also that these "hard limits" may still be high enough to
allow for excessive resource usage in a given use case. They are
compile time modifiable, so building your own library versions will
allow you to change the limits to your own needs. Also see the next
question.</p>
</div>
<div class="section" id="how-do-i-use-lxml-safely-as-a-web-service-endpoint">
<h2>How do I use lxml safely as a web-service endpoint?</h2>
<p>XML based web-service endpoints are generally subject to several
types of attacks if they allow some kind of untrusted input.
From the point of view of the underlying XML tool, the most
obvious attacks try to send a relatively small amount of data
that induces a comparatively large resource consumption on the
receiver side.</p>
<p>First of all, make sure network access is not enabled for the XML
parser that you use for parsing untrusted content and that it is
not configured to load external DTDs. Otherwise, attackers can
try to trick the parser into an attempt to load external resources
that are overly slow or impossible to retrieve, thus wasting time
and other valuable resources on your server such as socket
connections. Note that you can register your own document loader
in lxml, which allows for fine-grained control over any read access
to resources.</p>
<p>Some of the most famous excessive content expansion attacks
use XML entity references. Luckily, entity expansion is mostly
useless for the data commonly sent through web services and
can simply be disabled, which rules out several types of
denial of service attacks at once. This also involves an attack
that reads local files from the server, as XML entities can be
defined to expand into their content. Consequently, version
1.2 of the SOAP standard explicitly disallows entity references
in the XML stream.</p>
<p>To disable entity expansion, use an XML parser that is configured
with the option <tt class="docutils literal">resolve_entities=False</tt>. Then, after (or
while) parsing the document, use <tt class="docutils literal">root.iter(etree.Entity)</tt> to
recursively search for entity references. If it contains any,
reject the entire input document with a suitable error response.
In lxml 3.x, you can also use the new DTD introspection API to
apply your own restrictions on input documents.</p>
<p>Another attack to consider is compression bombs. If you allow
compressed input into your web service, attackers can try to send
well forged highly repetitive and thus very well compressing input
that unpacks into a very large XML document in your server's main
memory, potentially a thousand times larger than the compressed
input data.</p>
<p>As a counter measure, either disable compressed input for your
web server, at least for untrusted sources, or use incremental
parsing with <tt class="docutils literal">iterparse()</tt> instead of parsing the whole input
document into memory in one shot. That allows you to enforce
suitable limits on the input by applying semantic checks that
detect and prevent an illegitimate use of your service. If
possible, you can also use this to reduce the amount of data
that you need to keep in memory while parsing the document,
thus further reducing the possibility of an attacker to trick
your system into excessive resource usage.</p>
<p>Finally, please be aware that XPath suffers from the same
vulnerability as SQL when it comes to content injection. The
obvious fix is to not build any XPath expressions via string
formatting or concatenation when the parameters may come from
untrusted sources, and instead use XPath variables, which
safely expose their values to the evaluation engine.</p>
<p>The <a class="reference external" href="https://bitbucket.org/tiran/defusedxml">defusedxml</a> package comes with an example setup and a wrapper
API for lxml that applies certain counter measures internally.</p>
</div>
</div>
<div class="section" id="xpath-and-document-traversal">
<h1>XPath and Document Traversal</h1>
<div class="section" id="what-are-the-findall-and-xpath-methods-on-element-tree">
<h2>What are the <tt class="docutils literal">findall()</tt> and <tt class="docutils literal">xpath()</tt> methods on Element(Tree)?</h2>
<p><tt class="docutils literal">findall()</tt> is part of the original <a class="reference external" href="http://effbot.org/zone/element-index.htm">ElementTree API</a>. It supports a
<a class="reference external" href="http://effbot.org/zone/element-xpath.htm">simple subset of the XPath language</a>, without predicates, conditions and
other advanced features. It is very handy for finding specific tags in a
tree. Another important difference is namespace handling, which uses the
<tt class="docutils literal">{namespace}tagname</tt> notation. This is not supported by XPath. The
findall, find and findtext methods are compatible with other ElementTree
implementations and allow writing portable code that runs on ElementTree,
cElementTree and lxml.etree.</p>
<p><tt class="docutils literal">xpath()</tt>, on the other hand, supports the complete power of the XPath
language, including predicates, XPath functions and Python extension
functions. The syntax is defined by the <a class="reference external" href="http://www.w3.org/TR/xpath">XPath specification</a>. If you need
the expressiveness and selectivity of XPath, the <tt class="docutils literal">xpath()</tt> method, the
<tt class="docutils literal">XPath</tt> class and the <tt class="docutils literal">XPathEvaluator</tt> are the best <a class="reference external" href="performance.html#xpath">choice</a>.</p>
</div>
<div class="section" id="why-doesn-t-findall-support-full-xpath-expressions">
<h2>Why doesn't <tt class="docutils literal">findall()</tt> support full XPath expressions?</h2>
<p>It was decided that it is more important to keep compatibility with
<a class="reference external" href="http://effbot.org/zone/element-index.htm">ElementTree</a> to simplify code migration between the libraries. The main
difference compared to XPath is the <tt class="docutils literal">{namespace}tagname</tt> notation used in
<tt class="docutils literal">findall()</tt>, which is not valid XPath.</p>
<p>ElementTree and lxml.etree use the same implementation, which assures 100%
compatibility. Note that <tt class="docutils literal">findall()</tt> is <a class="reference external" href="performance.html#tree-traversal">so fast</a> in lxml that a native
implementation would not bring any performance benefits.</p>
</div>
<div class="section" id="how-can-i-find-out-which-namespace-prefixes-are-used-in-a-document">
<h2>How can I find out which namespace prefixes are used in a document?</h2>
<p>You can traverse the document (<tt class="docutils literal">root.iter()</tt>) and collect the prefix
attributes from all Elements into a set. However, it is unlikely that you
really want to do that. You do not need these prefixes, honestly. You only
need the namespace URIs. All namespace comparisons use these, so feel free to
make up your own prefixes when you use XPath expressions or extension
functions.</p>
<p>The only place where you might consider specifying prefixes is the
serialization of Elements that were created through the API. Here, you can
specify a prefix mapping through the <tt class="docutils literal">nsmap</tt> argument when creating the root
Element. Its children will then inherit this prefix for serialization.</p>
</div>
<div class="section" id="how-can-i-specify-a-default-namespace-for-xpath-expressions">
<h2>How can I specify a default namespace for XPath expressions?</h2>
<p>You can't. In XPath, there is no such thing as a default namespace. Just use
an arbitrary prefix and let the namespace dictionary of the XPath evaluators
map it to your namespace. See also the question above.</p>
</div>
</div>
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