Author: | Stefan Behnel |
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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.
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.
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>
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 >>> for child in root: ... print child.tag child1 child2 child3 >>> if root: ... print "root has children!" root has children! >>> 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
Note how 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']
To retrieve a 'real' Python list of all children (or a shallow copy of the element children list), you can call the getchildren() method:
>>> children = root.getchildren() >>> print type(children) is type([]) True >>> for child in children: ... print child.tag 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
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 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>
These two properties are enough to represent any text content in an XML document. If you want to read the text without the intermediate tags, however, you have to recursively concatenate all text and tail attributes in the correct order. A simpler way to do this is XPath:
>>> 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']
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.getiterator(): ... 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 getiterator() to have it filter for you:
>>> for element in root.getiterator("child"): ... print element.tag, '-', element.text child - Child 1 child - Child 2
In lxml.etree, elements provide further iterators for all directions in the tree: children, parents (or rather ancestors) and siblings.
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.
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) <ns0:html xmlns:ns0="http://www.w3.org/1999/xhtml"> <ns0:body>Hello World</ns0:body> </ns0: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 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: <spam&egg>.</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 >>> 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.