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modules in C</a></li></ul></li></ul></li></ul><ul id="Developing lxml"><li><span class="section title">Developing lxml</span><ul class="menu foreign" id="build"><li class="menu title"><a href="build.html">How to build lxml from source</a><ul class="submenu"><li class="menu item"><a href="build.html#cython">Cython</a></li><li class="menu item"><a href="build.html#subversion">Subversion</a></li><li class="menu item"><a href="build.html#setuptools">Setuptools</a></li><li class="menu item"><a href="build.html#running-the-tests-and-reporting-errors">Running the tests and reporting errors</a></li><li class="menu item"><a href="build.html#contributing-an-egg">Contributing an egg</a></li><li class="menu item"><a href="build.html#providing-newer-library-versions-on-mac-os-x">Providing newer library versions on Mac-OS X</a></li><li class="menu item"><a href="build.html#static-linking-on-windows">Static linking on Windows</a></li><li class="menu item"><a href="build.html#building-debian-packages-from-svn-sources">Building Debian packages from SVN sources</a></li></ul></li></ul><ul class="menu foreign" id="lxml source howto"><li class="menu title"><a href="lxml-source-howto.html">How to read the source of lxml</a><ul class="submenu"><li class="menu item"><a href="lxml-source-howto.html#what-is-cython?">What is Cython?</a></li><li 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-pyclasslookup">lxml.pyclasslookup</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 2 0 11"><li class="menu title"><a href="changes-2.0.11.html">Release Changelog</a></li></ul><ul class="menu foreign" id="credits"><li class="menu title"><a href="credits.html">Credits</a><ul class="submenu"><li class="menu item"><a href="credits.html#special-thanks-goes-to:">Special thanks goes to:</a></li></ul></li></ul></li></ul></div><h1 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" href="api/index.html">generated API
documentation</a>.</p>
<p>Separate pages describe the support for <a class="reference" href="parsing.html">parsing XML</a>, executing <a class="reference" href="xpathxslt.html">XPath and
XSLT</a>, <a class="reference" href="validation.html">validating XML</a> and interfacing with other XML tools through the
<a class="reference" href="sax.html">SAX-API</a>.</p>
<p>lxml is extremely extensible through <a class="reference" href="extensions.html">XPath functions in Python</a>, custom
<a class="reference" href="element_classes.html">Python element classes</a>, custom <a class="reference" href="resolvers.html">URL resolvers</a> and even <a class="reference" href="capi.html">at the C-level</a>.</p>
<div class="contents topic">
<p class="topic-title first"><a id="contents" name="contents">Contents</a></p>
<ul class="simple">
<li><a class="reference" href="#lxml-etree" id="id1" name="id1">lxml.etree</a></li>
<li><a class="reference" href="#other-element-apis" id="id2" name="id2">Other Element APIs</a></li>
<li><a class="reference" href="#trees-and-documents" id="id3" name="id3">Trees and Documents</a></li>
<li><a class="reference" href="#iteration" id="id4" name="id4">Iteration</a></li>
<li><a class="reference" href="#error-handling-on-exceptions" id="id5" name="id5">Error handling on exceptions</a></li>
<li><a class="reference" href="#error-logging" id="id6" name="id6">Error logging</a></li>
<li><a class="reference" href="#serialisation" id="id7" name="id7">Serialisation</a></li>
<li><a class="reference" href="#xinclude-and-elementinclude" id="id8" name="id8">XInclude and ElementInclude</a></li>
<li><a class="reference" href="#write-c14n-on-elementtree" id="id9" name="id9">write_c14n on ElementTree</a></li>
</ul>
</div>
<div class="section">
<h1><a id="lxml-etree" name="lxml-etree">lxml.etree</a></h1>
<p>lxml.etree tries to follow the <a class="reference" href="http://effbot.org/zone/element-index.htm">ElementTree API</a> wherever it can. There are
however some incompatibilities (see <a class="reference" 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"><span class="pre">lxml.etree.LXML_VERSION</span></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"><span class="pre">LIBXML_VERSION</span></tt> and <tt class="docutils literal"><span class="pre">LIBXSLT_VERSION</span></tt>.</p>
<p>The following examples usually assume this to be executed first:</p>
<pre class="literal-block">
>>> from lxml import etree
>>> from StringIO import StringIO
</pre>
</div>
<div class="section">
<h1><a id="other-element-apis" name="other-element-apis">Other Element APIs</a></h1>
<p>While lxml.etree itself uses the ElementTree API, it is possible to replace
the Element implementation by <a class="reference" href="namespace_extensions.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" href="objectify.html">objectify</a>, which is similar to
the <a class="reference" href="http://uche.ogbuji.net/tech/4suite/amara/">Amara bindery</a> tool.</p>
<p>lxml.etree comes with a number of <a class="reference" 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">
<h1><a id="trees-and-documents" name="trees-and-documents">Trees and Documents</a></h1>
<p>Compared to the original ElementTree API, lxml.etree has an extended tree
model. It knows about parents and siblings of elements:</p>
<pre class="literal-block">
>>> root = etree.Element("root")
>>> a = etree.SubElement(root, "a")
>>> b = etree.SubElement(root, "b")
>>> c = etree.SubElement(root, "c")
>>> d = etree.SubElement(root, "d")
>>> e = etree.SubElement(d, "e")
>>> b.getparent() == root
True
>>> print b.getnext().tag
c
>>> print c.getprevious().tag
b
</pre>
<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>
<pre class="literal-block">
>>> tree = d.getroottree()
>>> print tree.getroot().tag
root
</pre>
<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>
<pre class="literal-block">
>>> tree = etree.ElementTree(d)
>>> print tree.getroot().tag
d
>>> print etree.tostring(tree)
<d><e/></d>
</pre>
<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>
<pre class="literal-block">
>>> element = tree.getroot()
>>> print element.tag
d
>>> print element.getparent().tag
root
>>> print element.getroottree().getroot().tag
root
</pre>
<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">
<h1><a id="iteration" name="iteration">Iteration</a></h1>
<p>The ElementTree API makes Elements iterable to supports iteration over their
children. Using the tree defined above, we get:</p>
<pre class="literal-block">
>>> [ child.tag for child in root ]
['a', 'b', 'c', 'd']
</pre>
<p>To iterate in the opposite direction, use the <tt class="docutils literal"><span class="pre">reversed()</span></tt> function
that exists in Python 2.4 and later.</p>
<p>Tree traversal should use the <tt class="docutils literal"><span class="pre">element.iter()</span></tt> method:</p>
<pre class="literal-block">
>>> [ el.tag for el in root.iter() ]
['root', 'a', 'b', 'c', 'd', 'e']
</pre>
<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>
<pre class="literal-block">
>>> [ child.tag for child in root.iterchildren() ]
['a', 'b', 'c', 'd']
>>> [ child.tag for child in root.iterchildren(reversed=True) ]
['d', 'c', 'b', 'a']
>>> [ sibling.tag for sibling in b.itersiblings() ]
['c', 'd']
>>> [ sibling.tag for sibling in c.itersiblings(preceding=True) ]
['b', 'a']
>>> [ ancestor.tag for ancestor in e.iterancestors() ]
['d', 'root']
>>> [ el.tag for el in root.iterdescendants() ]
['a', 'b', 'c', 'd', 'e']
</pre>
<p>Note how <tt class="docutils literal"><span class="pre">element.iterdescendants()</span></tt> does not include the element
itself, as opposed to <tt class="docutils literal"><span class="pre">element.iter()</span></tt>. The latter effectively
implements the 'descendant-or-self' axis in XPath.</p>
<p>All of these iterators support an additional <tt class="docutils literal"><span class="pre">tag</span></tt> keyword argument that
filters the generated elements by tag name:</p>
<pre class="literal-block">
>>> [ child.tag for child in root.iterchildren(tag='a') ]
['a']
>>> [ child.tag for child in d.iterchildren(tag='a') ]
[]
>>> [ el.tag for el in root.iterdescendants(tag='d') ]
['d']
>>> [ el.tag for el in root.iter(tag='d') ]
['d']
</pre>
<p>See also the section on the utility functions <tt class="docutils literal"><span class="pre">iterparse()</span></tt> and
<tt class="docutils literal"><span class="pre">iterwalk()</span></tt> in the <a class="reference" href="parsing.html#iterparse-and-iterwalk">parser documentation</a>.</p>
</div>
<div class="section">
<h1><a id="error-handling-on-exceptions" name="error-handling-on-exceptions">Error handling on exceptions</a></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"><span class="pre">error_log</span></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 occured and "might have" lead to the problem from the error log
copy attached to the exception:</p>
<pre class="literal-block">
>>> etree.clearErrorLog()
>>> broken_xml = '''
... <root>
... <a>
... </root>
... '''
>>> try:
... etree.parse(StringIO(broken_xml))
... except etree.XMLSyntaxError, e:
... pass # just put the exception into e
</pre>
<p>Once you have caught this exception, you can access its <tt class="docutils literal"><span class="pre">error_log</span></tt> property
to retrieve the log entries or filter them by a specific type, error domain or
error level:</p>
<pre class="literal-block">
>>> log = e.error_log.filter_from_level(etree.ErrorLevels.FATAL)
>>> print log
<string>:4:8:FATAL:PARSER:ERR_TAG_NAME_MISMATCH: Opening and ending tag mismatch: a line 3 and root
<string>:5:1:FATAL:PARSER:ERR_TAG_NOT_FINISHED: Premature end of data in tag root line 2
</pre>
<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"><span class="pre">lxml.etree.ErrorDomains</span></tt> for that. You can get it from a log entry
like this:</p>
<pre class="literal-block">
>>> entry = log[0]
>>> print entry.domain_name, entry.type_name, entry.filename
PARSER ERR_TAG_NAME_MISMATCH <string>
</pre>
<p>There is also a convenience attribute <tt class="docutils literal"><span class="pre">last_error</span></tt> that returns the last
error or fatal error that occurred:</p>
<pre class="literal-block">
>>> entry = e.error_log.last_error
>>> print entry.domain_name, entry.type_name, entry.filename
PARSER ERR_TAG_NOT_FINISHED <string>
</pre>
</div>
<div class="section">
<h1><a id="error-logging" name="error-logging">Error logging</a></h1>
<p>lxml.etree supports logging libxml2 messages to the Python stdlib logging
module. This is done through the <tt class="docutils literal"><span class="pre">etree.PyErrorLog</span></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"><span class="pre">etree.useGlobalPythonLog</span></tt>
and the class <tt class="docutils literal"><span class="pre">etree.PyErrorLog</span></tt> for help. Note that this does not affect
the local error logs of XSLT, XMLSchema, etc.</p>
</div>
<div class="section">
<h1><a id="serialisation" name="serialisation">Serialisation</a></h1>
<p>lxml.etree has direct support for pretty printing XML output. Functions like
<tt class="docutils literal"><span class="pre">ElementTree.write()</span></tt> and <tt class="docutils literal"><span class="pre">tostring()</span></tt> support it through a keyword
argument:</p>
<pre class="literal-block">
>>> root = etree.XML("<root><test/></root>")
>>> print etree.tostring(root)
<root><test/></root>
>>> print etree.tostring(root, pretty_print=True),
<root>
<test/>
</root>
</pre>
<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>
<pre class="literal-block">
>>> unicode_root = etree.Element(u"t\u3120st")
>>> unicode_root.text = u"t\u0A0Ast"
>>> etree.tostring(unicode_root, encoding="utf-8")
'<t\xe3\x84\xa0st>t\xe0\xa8\x8ast</t\xe3\x84\xa0st>'
>>> print etree.tostring(unicode_root, encoding="iso-8859-1")
<?xml version='1.0' encoding='iso-8859-1'?>
<t&#12576;st>t&#2570;st</t&#12576;st>
</pre>
<p>Also see the general remarks on <a class="reference" 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>
<pre class="literal-block">
>>> print etree.tostring(root, xml_declaration=True)
<?xml version='1.0' encoding='ASCII'?>
<root><test/></root>
>>> unicode_root.clear()
>>> etree.tostring(unicode_root, encoding="UTF-16LE",
... xml_declaration=False)
'<\x00t\x00 1s\x00t\x00/\x00>\x00'
</pre>
<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>
<pre class="literal-block">
>>> notxml = etree.tostring(unicode_root, encoding="UTF-16LE",
... xml_declaration=False)
>>> root = etree.XML(notxml) #doctest: +ELLIPSIS
Traceback (most recent call last):
...
XMLSyntaxError: ...
</pre>
</div>
<div class="section">
<h1><a id="xinclude-and-elementinclude" name="xinclude-and-elementinclude">XInclude and ElementInclude</a></h1>
<p>You can let lxml process xinclude statements in a document by calling the
xinclude() method on a tree:</p>
<pre class="literal-block">
>>> data = StringIO('''\
... <doc xmlns:xi="http://www.w3.org/2001/XInclude">
... <foo/>
... <xi:include href="doc/test.xml" />
... </doc>''')
>>> tree = etree.parse(data)
>>> tree.xinclude()
>>> print etree.tostring(tree.getroot())
<doc xmlns:xi="http://www.w3.org/2001/XInclude">
<foo/>
<a xml:base="doc/test.xml"/>
</doc>
</pre>
<p>Note that the ElementTree compatible <a class="reference" href="http://effbot.org/zone/element-xinclude.htm">ElementInclude</a> module is also supported
as <tt class="docutils literal"><span class="pre">lxml.ElementInclude</span></tt>. It has the additional advantage of supporting
custom <a class="reference" 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">
<h1><a id="write-c14n-on-elementtree" name="write-c14n-on-elementtree">write_c14n on ElementTree</a></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>
<pre class="literal-block">
>>> f = StringIO('<a><b/></a>')
>>> tree = etree.parse(f)
>>> f2 = StringIO()
>>> tree.write_c14n(f2)
>>> f2.getvalue()
'<a><b></b></a>'
</pre>
</div>
</div>
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