The lxml.etree Tutorial

Author: Stefan Behnel

This tutorial briefly overviews the main concepts of the ElementTree API as implemented by lxml.etree, and some simple enhancements that make your life as a programmer easier.

For a complete reference of the API, see the generated API documentation.

Contents

A common way to import lxml.etree is as follows:

>>> from lxml import etree

If your code only uses the ElementTree API and does not rely on any functionality that is specific to lxml.etree, you can also use (any part of) the following import chain as a fall-back to the original ElementTree:

try:
  from lxml import etree
  print "running with lxml.etree"
except ImportError:
  try:
    # Python 2.5
    import xml.etree.cElementTree as etree
    print "running with cElementTree on Python 2.5+"
  except ImportError:
    try:
      # Python 2.5
      import xml.etree.ElementTree as etree
      print "running with ElementTree on Python 2.5+"
    except ImportError:
      try:
        # normal cElementTree install
        import cElementTree as etree
        print "running with cElementTree"
      except ImportError:
        try:
          # normal ElementTree install
          import elementtree.ElementTree as etree
          print "running with ElementTree"
        except ImportError:
          print "Failed to import ElementTree from any known place"

To aid in writing portable code, this tutorial makes it clear in the examples which part of the presented API is an extension of lxml.etree over the original ElementTree API, as defined by Fredrik Lundh's ElementTree library.

The Element class

An Element is the main container object for the ElementTree API. Most of the XML tree functionality is accessed through this class. Elements are easily created through the Element factory:

>>> root = etree.Element("root")

The XML tag name of elements is accessed through the tag property:

>>> print root.tag
root

Elements are organised in an XML tree structure. To create child elements and add them to a parent element, you can use the append() method:

>>> root.append( etree.Element("child1") )

However, this is so common that there is a shorter and much more efficient way to do this: the SubElement factory. It accepts the same arguments as the Element factory, but additionally requires the parent as first argument:

>>> child2 = etree.SubElement(root, "child2")
>>> child3 = etree.SubElement(root, "child3")

To see that this is really XML, you can serialise the tree you have created:

>>> print etree.tostring(root, pretty_print=True),
<root>
  <child1/>
  <child2/>
  <child3/>
</root>

Elements are lists

To make the access to these subelements as easy and straight forward as possible, elements behave like normal Python lists:

>>> child = root[0]
>>> print child.tag
child1

>>> print len(root)
3

>>> root.index(root[1]) # lxml.etree only!
1

>>> children = list(root)

>>> for child in root:
...     print child.tag
child1
child2
child3

>>> root.insert(0, etree.Element("child0"))
>>> start = root[:1]
>>> end   = root[-1:]

>>> print start[0].tag
child0
>>> print end[0].tag
child3

>>> root[0] = root[-1] # this moves the element!
>>> for child in root:
...     print child.tag
child3
child1
child2

Prior to ElementTree 1.3 and lxml 2.0, you could also check the truth value of an Element to see if it has children, i.e. if the list of children is empty. This is no longer supported as people tend to find it surprising that a non-None reference to an existing Element can evaluate to False. Instead, use len(element), which is both more explicit and less error prone.

Note in the examples that the last element was moved to a different position in the last example. This is a difference from the original ElementTree (and from lists), where elements can sit in multiple positions of any number of trees. In lxml.etree, elements can only sit in one position of one tree at a time.

If you want to copy an element to a different position, consider creating an independent deep copy using the copy module from Python's standard library:

>>> from copy import deepcopy

>>> element = etree.Element("neu")
>>> element.append( deepcopy(root[1]) )

>>> print element[0].tag
child1
>>> print [ c.tag for c in root ]
['child3', 'child1', 'child2']

The way up in the tree is provided through the getparent() method:

>>> root is root[0].getparent()  # lxml.etree only!
True

The siblings (or neighbours) of an element are accessed as next and previous elements:

>>> root[0] is root[1].getprevious() # lxml.etree only!
True
>>> root[1] is root[0].getnext() # lxml.etree only!
True

Elements carry attributes

XML elements support attributes. You can create them directly in the Element factory:

>>> root = etree.Element("root", interesting="totally")
>>> print etree.tostring(root)
<root interesting="totally"/>

Fast and direct access to these attributes is provided by the set() and get() methods of elements:

>>> print root.get("interesting")
totally

>>> root.set("interesting", "somewhat")
>>> print root.get("interesting")
somewhat

However, a very convenient way of dealing with them is through the dictionary interface of the attrib property:

>>> attributes = root.attrib

>>> print attributes["interesting"]
somewhat

>>> print attributes.get("hello")
None

>>> attributes["hello"] = "Guten Tag"
>>> print attributes.get("hello")
Guten Tag
>>> print root.get("hello")
Guten Tag

Elements contain text

Elements can contain text:

>>> root = etree.Element("root")
>>> root.text = "TEXT"

>>> print root.text
TEXT

>>> print etree.tostring(root)
<root>TEXT</root>

In many XML documents (data-centric documents), this is the only place where text can be found. It is encapsulated by a leaf tag at the very bottom of the tree hierarchy.

However, if XML is used for tagged text documents such as (X)HTML, text can also appear between different elements, right in the middle of the tree:

<html><body>Hello<br/>World</body></html>

Here, the <br/> tag is surrounded by text. This is often referred to as document-style or mixed-content XML. Elements support this through their tail property. It contains the text that directly follows the element, up to the next element in the XML tree:

>>> html = etree.Element("html")
>>> body = etree.SubElement(html, "body")
>>> body.text = "TEXT"

>>> print etree.tostring(html)
<html><body>TEXT</body></html>

>>> br = etree.SubElement(body, "br")
>>> print etree.tostring(html)
<html><body>TEXT<br/></body></html>

>>> br.tail = "TAIL"
>>> print etree.tostring(html)
<html><body>TEXT<br/>TAIL</body></html>

The two properties .text and .tail are enough to represent any text content in an XML document. This way, the ElementTree API does not require any special text nodes in addition to the Element class, that tend to get in the way fairly often (as you might know from classic DOM APIs).

However, there are cases where the tail text also gets in the way. For example, when you serialise an Element from within the tree, you do not always want its tail text in the result (although you would still want the tail text of its children). For this purpose, the tostring() function accepts the keyword argument with_tail:

>>> print etree.tostring(br)
<br/>TAIL
>>> print etree.tostring(br, with_tail=False) # lxml.etree only!
<br/>

If you want to read only the text, i.e. without any intermediate tags, you have to recursively concatenate all text and tail attributes in the correct order. Again, the tostring() function comes to the rescue, this time using the method keyword:

>>> print etree.tostring(html, method="text")
TEXTTAIL

Using XPath to find text

Another way to extract the text content of a tree is XPath, which also allows you to extract the separate text chunks into a list:

>>> print html.xpath("string()") # lxml.etree only!
TEXTTAIL
>>> print html.xpath("//text()") # lxml.etree only!
['TEXT', 'TAIL']

If you want to use this more often, you can wrap it in a function:

>>> build_text_list = etree.XPath("//text()") # lxml.etree only!
>>> print build_text_list(html)
['TEXT', 'TAIL']

Note that a string result returned by XPath is a special 'smart' object that knows about its origins. You can ask it where it came from through its getparent() method, just as you would with Elements:

>>> texts = build_text_list(html)
>>> print texts[0]
TEXT
>>> parent = texts[0].getparent()
>>> print parent.tag
body

>>> print texts[1]
TAIL
>>> print texts[1].getparent().tag
br

You can also find out if it's normal text content or tail text:

>>> print texts[0].is_text
True
>>> print texts[1].is_text
False
>>> print texts[1].is_tail
True

While this works for the results of the text() function, lxml will not to tell you the origin of a string value that was constructed by the XPath functions string() or concat():

>>> stringify = etree.XPath("string()")
>>> print stringify(html)
TEXTTAIL
>>> print stringify(html).getparent()
None

Tree iteration

For problems like the above, where you want to recursively traverse the tree and do something with its elements, tree iteration is a very convenient solution. Elements provide a tree iterator for this purpose. It yields elements in document order, i.e. in the order their tags would appear if you serialised the tree to XML:

>>> root = etree.Element("root")
>>> etree.SubElement(root, "child").text = "Child 1"
>>> etree.SubElement(root, "child").text = "Child 2"
>>> etree.SubElement(root, "another").text = "Child 3"

>>> print etree.tostring(root, pretty_print=True),
<root>
  <child>Child 1</child>
  <child>Child 2</child>
  <another>Child 3</another>
</root>

>>> for element in root.iter():
...     print element.tag, '-', element.text
root - None
child - Child 1
child - Child 2
another - Child 3

If you know you are only interested in a single tag, you can pass its name to iter() to have it filter for you:

>>> for element in root.iter("child"):
...     print element.tag, '-', element.text
child - Child 1
child - Child 2

By default, iteration yields all nodes in the tree, including ProcessingInstructions, Comments and Entity instances. If you want to make sure only Element objects are returned, you can pass the Element factory as tag parameter:

>>> root.append(etree.Entity("#234"))
>>> root.append(etree.Comment("some comment"))

>>> for element in root.iter():
...     if isinstance(element.tag, basestring):
...         print element.tag, '-', element.text
...     else:
...         print 'SPECIAL:', element, '-', element.text
root - None
child - Child 1
child - Child 2
another - Child 3
SPECIAL: &#234; - &#234;
SPECIAL: <!--some comment--> - some comment

>>> for element in root.iter(tag=etree.Element):
...     print element.tag, '-', element.text
root - None
child - Child 1
child - Child 2
another - Child 3

>>> for element in root.iter(tag=etree.Entity):
...     print element.text
&#234;

In lxml.etree, elements provide further iterators for all directions in the tree: children, parents (or rather ancestors) and siblings.

Serialisation

Serialisation commonly uses the tostring() function that returns a string, or the ElementTree.write() method that writes to a file or file-like object. Both accept the same keyword arguments like pretty_print for formatted output or encoding to select a specific output encoding other than plain ASCII:

>>> root = etree.XML('<root><a><b/></a></root>')

>>> print etree.tostring(root)
<root><a><b/></a></root>

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

>>> print etree.tostring(root, encoding='iso-8859-1')
<?xml version='1.0' encoding='iso-8859-1'?>
<root><a><b/></a></root>

>>> print etree.tostring(root, pretty_print=True),
<root>
  <a>
    <b/>
  </a>
</root>

Note the newline that is appended at the end when pretty printing the output.

Since lxml 2.0 (and ElementTree 1.3), the serialisation functions can do more than XML serialisation. You can serialise to HTML or extract the text content by passing the method keyword:

>>> root = etree.XML('<html><head/><body><p>Hello<br/>World</p></body></html>')

>>> print etree.tostring(root) # default: method = 'xml'
<html><head/><body><p>Hello<br/>World</p></body></html>

>>> print etree.tostring(root, method='xml') # same as above
<html><head/><body><p>Hello<br/>World</p></body></html>

>>> print etree.tostring(root, method='html')
<html><head></head><body><p>Hello<br>World</p></body></html>

>>> print etree.tostring(root, method='html', pretty_print=True),
<html>
<head></head>
<body><p>Hello<br>World</p></body>
</html>

>>> print etree.tostring(root, method='text')
HelloWorld

For the plain text output, serialising to a Python unicode string might become handy. Just pass the unicode type as encoding:

>>> etree.tostring(root, encoding=unicode, method='text')
u'HelloWorld'

The ElementTree class

An ElementTree is mainly a document wrapper around a tree with a root node. It provides a couple of methods for parsing, serialisation and general document handling. One of the bigger differences is that it serialises as a complete document, as opposed to a single Element. This includes top-level processing instructions and comments, as well as a DOCTYPE and other DTD content in the document:

>>> from StringIO import StringIO
>>> tree = etree.parse(StringIO('''\
... <?xml version="1.0"?>
... <!DOCTYPE root SYSTEM "test" [ <!ENTITY tasty "eggs"> ]>
... <root>
...   <a>&tasty;</a>
... </root>
... '''))

>>> print tree.docinfo.doctype
<!DOCTYPE root SYSTEM "test">

>>> # lxml 1.3.4 and later
>>> print etree.tostring(tree)
<!DOCTYPE root SYSTEM "test" [
<!ENTITY tasty "eggs">
]>
<root>
  <a>eggs</a>
</root>

>>> # lxml 1.3.4 and later
>>> print etree.tostring(etree.ElementTree(tree.getroot()))
<!DOCTYPE root SYSTEM "test" [
<!ENTITY tasty "eggs">
]>
<root>
  <a>eggs</a>
</root>

>>> # ElementTree and lxml <= 1.3.3
>>> print etree.tostring(tree.getroot())
<root>
  <a>eggs</a>
</root>

Note that this has changed in lxml 1.3.4 to match the behaviour of the upcoming lxml 2.0. Before, both would serialise without DTD content, which made lxml loose DTD information in an input-output cycle.

Parsing from strings and files

lxml.etree supports parsing XML in a number of ways and from all important sources, namely strings, files, URLs (http/ftp) and file-like objects. The main parse functions are fromstring() and parse(), both called with the source as first argument. By default, they use the standard parser, but you can always pass a different parser as second argument.

The fromstring() function

The fromstring() function is the easiest way to parse a string:

>>> some_xml_data = "<root>data</root>"

>>> root = etree.fromstring(some_xml_data)
>>> print root.tag
root
>>> print etree.tostring(root)
<root>data</root>

The XML() function

The XML() function behaves like the fromstring() function, but is commonly used to write XML literals right into the source:

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

The parse() function

The parse() function is used to parse from files and file-like objects:

>>> some_file_like = StringIO("<root>data</root>")

>>> tree = etree.parse(some_file_like)

>>> print etree.tostring(tree)
<root>data</root>

Note that parse() returns an ElementTree object, not an Element object as the string parser functions:

>>> root = tree.getroot()
>>> print root.tag
root
>>> print etree.tostring(root)
<root>data</root>

The reasoning behind this difference is that parse() returns a complete document from a file, while the string parsing functions are commonly used to parse XML fragments.

The parse() function supports any of the following sources:

  • an open file object
  • a file-like object that has a .read(byte_count) method returning a byte string on each call
  • a filename string
  • an HTTP or FTP URL string

Note that passing a filename or URL is usually faster than passing an open file.

Parser objects

By default, lxml.etree uses a standard parser with a default setup. If you want to configure the parser, you can create a you instance:

>>> parser = etree.XMLParser(remove_blank_text=True) # lxml.etree only!

This creates a parser that removes empty text between tags while parsing, which can reduce the size of the tree and avoid dangling tail text if you know that whitespace-only content is not meaningful for your data. An example:

>>> root = etree.XML("<root>  <a/>   <b>  </b>     </root>", parser)

>>> print etree.tostring(root)
<root><a/><b>  </b></root>

Note that the whitespace content inside the <b> tag was not removed, as content at leaf elements tends to be data content (even if blank). You can easily remove it in an additional step by traversing the tree:

>>> for element in root.iter("*"):
...     if element.text is not None and not element.text.strip():
...         element.text = None

>>> print etree.tostring(root)
<root><a/><b/></root>

See help(etree.XMLParser) to find out about the available parser options.

Incremental parsing

lxml.etree provides two ways for incremental step-by-step parsing. One is through file-like objects, where it calls the read() method repeatedly. This is best used where the data arrives from a source like urllib or any other file-like object that can provide data on request. Note that the parser will block and wait until data becomes available in this case:

>>> class DataSource:
...     data = iter(["<roo", "t><", "a/", "><", "/root>"])
...     def read(self, requested_size):
...         try:
...             return self.data.next()
...         except StopIteration:
...             return ""

>>> tree = etree.parse(DataSource())

>>> print etree.tostring(tree)
<root><a/></root>

The second way is through a feed parser interface, given by the feed(data) and close() methods:

>>> parser = etree.XMLParser()

>>> parser.feed("<roo")
>>> parser.feed("t><")
>>> parser.feed("a/")
>>> parser.feed("><")
>>> parser.feed("/root>")

>>> root = parser.close()

>>> print etree.tostring(root)
<root><a/></root>

Here, you can interrupt the parsing process at any time and continue it later on with another call to the feed() method. This comes in handy if you want to avoid blocking calls to the parser, e.g. in frameworks like Twisted, or whenever data comes in slowly or in chunks and you want to do other things while waiting for the next chunk.

After calling the close() method (or when an exception was raised by the parser), you can reuse the parser by calling its feed() method again:

>>> parser.feed("<root/>")
>>> root = parser.close()
>>> print etree.tostring(root)
<root/>

Event-driven parsing

Sometimes, all you need from a document is a small fraction somewhere deep inside the tree, so parsing the whole tree into memory, traversing it and dropping it can be too much overhead. lxml.etree supports this use case with two event-driven parser interfaces, one that generates parser events while building the tree (iterparse), and one that does not build the tree at all, and instead calls feedback methods on a target object in a SAX-like fashion.

Here is a simple iterparse() example:

>>> some_file_like = StringIO("<root><a>data</a></root>")

>>> for event, element in etree.iterparse(some_file_like):
...     print "%s, %4s, %s" % (event, element.tag, element.text)
end,    a, data
end, root, None

By default, iterparse() only generates events when it is done parsing an element, but you can control this through the events keyword argument:

>>> some_file_like = StringIO("<root><a>data</a></root>")

>>> for event, element in etree.iterparse(some_file_like,
...                                       events=("start", "end")):
...     print "%5s, %4s, %s" % (event, element.tag, element.text)
start, root, None
start,    a, data
  end,    a, data
  end, root, None

Note that the text, tail and children of an Element are not necessarily there yet when receiving the start event. Only the end event guarantees that the Element has been parsed completely.

It also allows to .clear() or modify the content of an Element to save memory. So if you parse a large tree and you want to keep memory usage small, you should clean up parts of the tree that you no longer need:

>>> some_file_like = StringIO(
...     "<root><a><b>data</b></a><a><b/></a></root>")

>>> for event, element in etree.iterparse(some_file_like):
...     if element.tag == 'b':
...         print element.text
...     elif element.tag == 'a':
...         print "** cleaning up the subtree"
...         element.clear()
data
** cleaning up the subtree
None
** cleaning up the subtree

If memory is a real bottleneck, or if building the tree is not desired at all, the target parser interface of lxml.etree can be used. It creates SAX-like events by calling the methods of a target object. By implementing some or all of these methods, you can control which events are generated:

>>> class ParserTarget:
...     events = []
...     def start(self, tag, attrib):
...         self.events.append(("start", tag, attrib))
...     def close(self):
...         return self.events

>>> parser = etree.XMLParser(target=ParserTarget())
>>> events = etree.fromstring('<root test="true"/>', parser)

>>> for event in events:
...     print 'event: %s - tag: %s' % (event[0], event[1])
...     for attr, value in event[2].iteritems():
...         print ' * %s = %s' % (attr, value)
event: start - tag: root
 * test = true

Namespaces

The ElementTree API avoids namespace prefixes wherever possible and deploys the real namespaces instead:

>>> xhtml = etree.Element("{http://www.w3.org/1999/xhtml}html")
>>> body = etree.SubElement(xhtml, "{http://www.w3.org/1999/xhtml}body")
>>> body.text = "Hello World"

>>> print etree.tostring(xhtml, pretty_print=True),
<html:html xmlns:html="http://www.w3.org/1999/xhtml">
  <html:body>Hello World</html:body>
</html:html>

As you can see, prefixes only become important when you serialise the result. However, the above code becomes somewhat verbose due to the lengthy namespace names. And retyping or copying a string over and over again is error prone. It is therefore common practice to store a namespace URI in a global variable. To adapt the namespace prefixes for serialisation, you can also pass a mapping to the Element factory, e.g. to define the default namespace:

>>> XHTML_NAMESPACE = "http://www.w3.org/1999/xhtml"
>>> XHTML = "{%s}" % XHTML_NAMESPACE

>>> NSMAP = {None : XHTML_NAMESPACE} # the default namespace (no prefix)

>>> xhtml = etree.Element(XHTML + "html", nsmap=NSMAP) # lxml only!
>>> body = etree.SubElement(xhtml, XHTML + "body")
>>> body.text = "Hello World"

>>> print etree.tostring(xhtml, pretty_print=True),
<html xmlns="http://www.w3.org/1999/xhtml">
  <body>Hello World</body>
</html>

Namespaces on attributes work alike:

>>> body.set(XHTML + "bgcolor", "#CCFFAA")

>>> print etree.tostring(xhtml, pretty_print=True),
<html xmlns="http://www.w3.org/1999/xhtml">
  <body bgcolor="#CCFFAA">Hello World</body>
</html>

>>> print body.get("bgcolor")
None
>>> body.get(XHTML + "bgcolor")
'#CCFFAA'

You can also use XPath in this way:

>>> find_xhtml_body = etree.ETXPath(      # lxml only !
...     "//{%s}body" % XHTML_NAMESPACE)
>>> results = find_xhtml_body(xhtml)

>>> print results[0].tag
{http://www.w3.org/1999/xhtml}body

The E-factory

The E-factory provides a simple and compact syntax for generating XML and HTML:

>>> from lxml.builder import E

>>> def CLASS(*args): # class is a reserved word in Python
...     return {"class":' '.join(args)}

>>> html = page = (
...   E.html(       # create an Element called "html"
...     E.head(
...       E.title("This is a sample document")
...     ),
...     E.body(
...       E.h1("Hello!", CLASS("title")),
...       E.p("This is a paragraph with ", E.b("bold"), " text in it!"),
...       E.p("This is another paragraph, with a", "\n      ",
...         E.a("link", href="http://www.python.org"), "."),
...       E.p("Here are some reservered characters: <spam&egg>."),
...       etree.XML("<p>And finally an embedded XHTML fragment.</p>"),
...     )
...   )
... )

>>> print etree.tostring(page, pretty_print=True),
<html>
  <head>
    <title>This is a sample document</title>
  </head>
  <body>
    <h1 class="title">Hello!</h1>
    <p>This is a paragraph with <b>bold</b> text in it!</p>
    <p>This is another paragraph, with a
      <a href="http://www.python.org">link</a>.</p>
    <p>Here are some reservered characters: &lt;spam&amp;egg&gt;.</p>
    <p>And finally an embedded XHTML fragment.</p>
  </body>
</html>

The Element creation based on attribute access makes it easy to build up a simple vocabulary for an XML language:

>>> from lxml.builder import ElementMaker # lxml only !

>>> E = ElementMaker(namespace="http://my.de/fault/namespace",
...                  nsmap={'p' : "http://my.de/fault/namespace"})

>>> DOC = E.doc
>>> TITLE = E.title
>>> SECTION = E.section
>>> PAR = E.par

>>> my_doc = DOC(
...   TITLE("The dog and the hog"),
...   SECTION(
...     TITLE("The dog"),
...     PAR("Once upon a time, ..."),
...     PAR("And then ...")
...   ),
...   SECTION(
...     TITLE("The hog"),
...     PAR("Sooner or later ...")
...   )
... )

>>> print etree.tostring(my_doc, pretty_print=True),
<p:doc xmlns:p="http://my.de/fault/namespace">
  <p:title>The dog and the hog</p:title>
  <p:section>
    <p:title>The dog</p:title>
    <p:par>Once upon a time, ...</p:par>
    <p:par>And then ...</p:par>
  </p:section>
  <p:section>
    <p:title>The hog</p:title>
    <p:par>Sooner or later ...</p:par>
  </p:section>
</p:doc>

One such example is the module lxml.html.builder, which provides a vocabulary for HTML.

ElementPath

The ElementTree library comes with a simple XPath-like path language called ElementPath. The main difference is that you can use the {namespace}tag notation in ElementPath expressions. However, advanced features like value comparison and functions are not available.

In addition to a full XPath implementation, lxml.etree supports the ElementPath language in the same way ElementTree does, even using (almost) the same implementation. The API provides four methods here that you can find on Elements and ElementTrees:

Here are some examples:

>>> root = etree.XML("<root><a x='123'>aText<b/><c/><b/></a></root>")

Find a child of an Element:

>>> print root.find("b")
None
>>> print root.find("a").tag
a

Find an Element anywhere in the tree:

>>> print root.find(".//b").tag
b
>>> [ b.tag for b in root.iterfind(".//b") ]
['b', 'b']

Find Elements with a certain attribute:

>>> print root.findall(".//a[@x]")[0].tag
a
>>> print root.findall(".//a[@y]")
[]
Generated on: 2008-12-12.