APIs specific to lxml.etree

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.

Separate pages describe the support for parsing XML, executing XPath and XSLT, validating XML and interfacing with other XML tools through the SAX-API.

lxml is extremely extensible through XPath functions in Python, custom Python element classes, custom URL resolvers and even at the C-level.



lxml.etree tries to follow the ElementTree API wherever it can. There are however some incompatibilities (see compatibility). The extensions are documented here.

If you need to know which version of lxml is installed, you can access the lxml.etree.LXML_VERSION 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 LIBXML_VERSION and LIBXSLT_VERSION.

The following examples usually assume this to be executed first:

>>> from lxml import etree
>>> from StringIO import StringIO

Other Element APIs

While lxml.etree itself uses the ElementTree API, it is possible to replace the Element implementation by custom element subclasses. This has been used to implement well-known XML APIs on top of lxml. For example, lxml ships with a data-binding implementation called objectify, which is similar to the Amara bindery tool.

lxml.etree comes with a number of different lookup schemes to customize the mapping between libxml2 nodes and the Element classes used by lxml.etree.

Trees and Documents

Compared to the original ElementTree API, lxml.etree has an extended tree model. It knows about parents and siblings of elements:

>>> 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
>>> print b.getnext().tag
>>> print c.getprevious().tag

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:

>>> tree = d.getroottree()
>>> print tree.getroot().tag

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:

>>> tree = etree.ElementTree(d)
>>> print tree.getroot().tag
>>> print etree.tostring(tree)

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:

>>> element = tree.getroot()
>>> print element.tag
>>> print element.getparent().tag
>>> print element.getroottree().getroot().tag

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.


The ElementTree API makes Elements iterable to supports iteration over their children. Using the tree defined above, we get:

>>> [ el.tag for el in root ]
['a', 'b', 'c', 'd']

Tree traversal is commonly based on the element.getiterator() method:

>>> [ el.tag for el in root.getiterator() ]
['root', 'a', 'b', 'c', 'd', 'e']

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:

>>> [ el.tag for el in root.iterchildren() ]
['a', 'b', 'c', 'd']
>>> [ el.tag for el in root.iterchildren(reversed=True) ]
['d', 'c', 'b', 'a']
>>> [ el.tag for el in b.itersiblings() ]
['c', 'd']
>>> [ el.tag for el in c.itersiblings(preceding=True) ]
['b', 'a']
>>> [ el.tag for el in e.iterancestors() ]
['d', 'root']
>>> [ el.tag for el in root.iterdescendants() ]
['a', 'b', 'c', 'd', 'e']

Note how element.iterdescendants() does not include the element itself, as opposed to element.getiterator(). The latter effectively implements the 'descendant-or-self' axis in XPath.

All of these iterators support an additional tag keyword argument that filters the generated elements by tag name:

>>> [ el.tag for el in root.iterchildren(tag='a') ]
>>> [ el.tag for el in d.iterchildren(tag='a') ]
>>> [ el.tag for el in root.iterdescendants(tag='d') ]
>>> [ el.tag for el in root.getiterator(tag='d') ]

See also the section on the utility functions iterparse() and iterwalk() in the parser documentation.

Error handling on exceptions

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 error_log property of the respective evaluator or transformer object. See their documentation for details.

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:

>>> etree.clearErrorLog()
>>> broken_xml = '''
... <root>
...   <a>
... </root>
... '''
>>> try:
...   etree.parse(StringIO(broken_xml))
... except etree.XMLSyntaxError, e:
...   pass # just put the exception into e

Once you have caught this exception, you can access its error_log property to retrieve the log entries or filter them by a specific type, error domain or error level:

>>> log = e.error_log.filter_from_level(etree.ErrorLevels.FATAL)
>>> print log
<string>:4:FATAL:PARSER:ERR_TAG_NAME_MISMATCH: Opening and ending tag mismatch: a line 3 and root
<string>:5:FATAL:PARSER:ERR_TAG_NOT_FINISHED: Premature end of data in tag root line 2

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 and 5 of a string (<string>, or the filename if available). Here, PARSER is the so-called error domain, see lxml.etree.ErrorDomains for that. You can get it from a log entry like this:

>>> entry = log[0]
>>> print entry.domain_name, entry.type_name, entry.filename

There is also a convenience attribute last_error that returns the last error or fatal error that occurred:

>>> entry = e.error_log.last_error
>>> print entry.domain_name, entry.type_name, entry.filename

Error logging

lxml.etree supports logging libxml2 messages to the Python stdlib logging module. This is done through the etree.PyErrorLog 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 etree.useGlobalPythonLog and the class etree.PyErrorLog for help. Note that this does not affect the local error logs of XSLT, XMLSchema, etc.


lxml.etree has direct support for pretty printing XML output. Functions like ElementTree.write() and tostring() support it through a keyword argument:

>>> root = etree.XML("<root><test/></root>")
>>> print etree.tostring(root)

>>> print etree.tostring(root, pretty_print=True)

By default, lxml (just as ElementTree) outputs the XML declaration only if it is required by the standard:

>>> unicode_root = etree.Element(u"t\u1234st")
>>> unicode_root.text = u"t\u4321st"
>>> etree.tostring(unicode_root, encoding="utf-8")

>>> print etree.tostring(unicode_root, encoding="iso-8859-1")
<?xml version='1.0' encoding='iso-8859-1'?>

Also see the general remarks on Unicode support.

You can enable or disable the declaration explicitly by passing another keyword argument for the serialisation:

>>> print etree.tostring(root, xml_declaration=True)
<?xml version='1.0' encoding='ASCII'?>

>>> etree.tostring(unicode_root, encoding="utf-8",
...                              xml_declaration=False)

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:

>>> notxml = etree.tostring(unicode_root, encoding="utf-8",
...                                       xml_declaration=False)
>>> etree.XML(notxml)
Traceback (most recent call last):
XMLSyntaxError: line 1: Extra content at the end of the document

XInclude and ElementInclude

You can let lxml process xinclude statements in a document by calling the xinclude() method on a tree:

>>> 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">
<a xml:base="doc/test.xml"/>

Note that the ElementTree compatible ElementInclude module is also supported as lxml.ElementInclude. It has the additional advantage of supporting custom URL resolvers 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.

write_c14n on ElementTree

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:

>>> f = StringIO('<a><b/></a>')
>>> tree = etree.parse(f)
>>> f2 = StringIO()
>>> tree.write_c14n(f2)
>>> f2.getvalue()