Parsing XML and HTML with lxml

lxml provides a very simple and powerful API for parsing XML and HTML. It supports one-step parsing as well as step-by-step parsing using an event-driven API (currently only for XML).

Contents

The usual setup procedure:

>>> from lxml import etree

Parsers

Parsers are represented by parser objects. There is support for parsing both XML and (broken) HTML. Note that XHTML is best parsed as XML, parsing it with the HTML parser can lead to unexpected results. Here is a simple example for parsing XML from an in-memory string:

>>> xml = '<a xmlns="test"><b xmlns="test"/></a>'

>>> root = etree.fromstring(xml)
>>> etree.tostring(root)
b'<a xmlns="test"><b xmlns="test"/></a>'

To read from a file or file-like object, you can use the parse() function, which returns an ElementTree object:

>>> tree = etree.parse(StringIO(xml))
>>> etree.tostring(tree.getroot())
b'<a xmlns="test"><b xmlns="test"/></a>'

Note how the parse() function reads from a file-like object here. If parsing is done from a real file, it is more common (and also somewhat more efficient) to pass a filename:

>>> tree = etree.parse("doc/test.xml")

lxml can parse from a local file, an HTTP URL or an FTP URL. It also auto-detects and reads gzip-compressed XML files (.gz).

If you want to parse from memory and still provide a base URL for the document (e.g. to support relative paths in an XInclude), you can pass the base_url keyword argument:

>>> root = etree.fromstring(xml, base_url="http://where.it/is/from.xml")

Parser options

The parsers accept a number of setup options as keyword arguments. The above example is easily extended to clean up namespaces during parsing:

>>> parser = etree.XMLParser(ns_clean=True)
>>> tree   = etree.parse(StringIO(xml), parser)
>>> etree.tostring(tree.getroot())
b'<a xmlns="test"><b/></a>'

The keyword arguments in the constructor are mainly based on the libxml2 parser configuration. A DTD will also be loaded if validation or attribute default values are requested.

Available boolean keyword arguments:

  • attribute_defaults - read the DTD (if referenced by the document) and add the default attributes from it
  • dtd_validation - validate while parsing (if a DTD was referenced)
  • load_dtd - load and parse the DTD while parsing (no validation is performed)
  • no_network - prevent network access when looking up external documents (on by default)
  • ns_clean - try to clean up redundant namespace declarations
  • recover - try hard to parse through broken XML
  • remove_blank_text - discard blank text nodes between tags
  • remove_comments - discard comments
  • remove_pis - discard processing instructions
  • strip_cdata - replace CDATA sections by normal text content (on by default)
  • resolve_entities - replace entities by their text value (on by default)
  • huge_tree - disable security restrictions and support very deep trees and very long text content (only affects libxml2 2.7+)
  • compact - use compact storage for short text content (on by default)

Error log

Parsers have an error_log property that lists the errors of the last parser run:

>>> parser = etree.XMLParser()
>>> print(len(parser.error_log))
0

>>> tree = etree.XML("<root></b>", parser)
Traceback (most recent call last):
  ...
lxml.etree.XMLSyntaxError: Opening and ending tag mismatch: root line 1 and b, line 1, column 11

>>> print(len(parser.error_log))
1

>>> error = parser.error_log[0]
>>> print(error.message)
Opening and ending tag mismatch: root line 1 and b
>>> print(error.line)
1
>>> print(error.column)
11

Parsing HTML

HTML parsing is similarly simple. The parsers have a recover keyword argument that the HTMLParser sets by default. It lets libxml2 try its best to return a valid HTML tree with all content it can manage to parse. It will not raise an exception on parser errors. You should use libxml2 version 2.6.21 or newer to take advantage of this feature.

>>> broken_html = "<html><head><title>test<body><h1>page title</h3>"

>>> parser = etree.HTMLParser()
>>> tree   = etree.parse(StringIO(broken_html), parser)

>>> result = etree.tostring(tree.getroot(),
...                         pretty_print=True, method="html")
>>> print(result)
<html>
  <head>
    <title>test</title>
  </head>
  <body>
    <h1>page title</h1>
  </body>
</html>

Lxml has an HTML function, similar to the XML shortcut known from ElementTree:

>>> html = etree.HTML(broken_html)
>>> result = etree.tostring(html, pretty_print=True, method="html")
>>> print(result)
<html>
  <head>
    <title>test</title>
  </head>
  <body>
    <h1>page title</h1>
  </body>
</html>

The support for parsing broken HTML depends entirely on libxml2's recovery algorithm. It is not the fault of lxml if you find documents that are so heavily broken that the parser cannot handle them. There is also no guarantee that the resulting tree will contain all data from the original document. The parser may have to drop seriously broken parts when struggling to keep parsing. Especially misplaced meta tags can suffer from this, which may lead to encoding problems.

Note that the result is a valid HTML tree, but it may not be a well-formed XML tree. For example, XML forbids double hyphens in comments, which the HTML parser will happily accept in recovery mode. Therefore, if your goal is to serialise an HTML document as an XML/XHTML document after parsing, you may have to apply some manual preprocessing first.

Doctype information

The use of the libxml2 parsers makes some additional information available at the API level. Currently, ElementTree objects can access the DOCTYPE information provided by a parsed document, as well as the XML version and the original encoding:

>>> pub_id  = "-//W3C//DTD XHTML 1.0 Transitional//EN"
>>> sys_url = "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
>>> doctype_string = '<!DOCTYPE html PUBLIC "%s" "%s">' % (pub_id, sys_url)
>>> xml_header = '<?xml version="1.0" encoding="ascii"?>'
>>> xhtml = xml_header + doctype_string + '<html><body></body></html>'

>>> tree = etree.parse(StringIO(xhtml))
>>> docinfo = tree.docinfo
>>> print(docinfo.public_id)
-//W3C//DTD XHTML 1.0 Transitional//EN
>>> print(docinfo.system_url)
http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd
>>> docinfo.doctype == doctype_string
True

>>> print(docinfo.xml_version)
1.0
>>> print(docinfo.encoding)
ascii

The target parser interface

As in ElementTree, and similar to a SAX event handler, you can pass a target object to the parser:

>>> class EchoTarget:
...     def start(self, tag, attrib):
...         print("start %s %s" % (tag, attrib))
...     def end(self, tag):
...         print("end %s" % tag)
...     def data(self, data):
...         print("data %r" % data)
...     def comment(self, text):
...         print("comment %s" % text)
...     def close(self):
...         print("close")
...         return "closed!"

>>> parser = etree.XMLParser(target = EchoTarget())

>>> result = etree.XML("<element>some<!--comment-->text</element>",
...                    parser)
start element {}
data u'some'
comment comment
data u'text'
end element
close

>>> print(result)
closed!

It is important for the .close() method to reset the parser target to a usable state, so that you can reuse the parser as often as you like:

>>> result = etree.XML("<element>some<!--comment-->text</element>",
...                    parser)
start element {}
data u'some'
comment comment
data u'text'
end element
close

>>> print(result)
closed!

Note that the parser does not build a tree when using a parser target. The result of the parser run is whatever the target object returns from its .close() method. If you want to return an XML tree here, you have to create it programmatically in the target object. An example for a parser target that builds a tree is the TreeBuilder.

>>> parser = etree.XMLParser(target = etree.TreeBuilder())

>>> result = etree.XML("<element>some<!--comment-->text</element>",
...                    parser)

>>> print(result.tag)
element
>>> print(result[0].text)
comment

The feed parser interface

Since lxml 2.0, the parsers have a feed parser interface that is compatible to the ElementTree parsers. You can use it to feed data into the parser in a controlled step-by-step way.

In lxml.etree, you can use both interfaces to a parser at the same time: the parse() or XML() functions, and the feed parser interface. Both are independent and will not conflict (except if used in conjunction with a parser target object as described above).

To start parsing with a feed parser, just call its feed() method to feed it some data.

>>> parser = etree.XMLParser()

>>> for data in ('<?xml versio', 'n="1.0"?', '><roo', 't><a', '/></root>'):
...     parser.feed(data)

When you are done parsing, you must call the close() method to retrieve the root Element of the parse result document, and to unlock the parser:

>>> root = parser.close()

>>> print(root.tag)
root
>>> print(root[0].tag)
a

If you do not call close(), the parser will stay locked and subsequent feeds will keep appending data, usually resulting in a non well-formed document and an unexpected parser error. So make sure you always close the parser after use, also in the exception case.

Another way of achieving the same step-by-step parsing is by writing your own file-like object that returns a chunk of data on each read() call. Where the feed parser interface allows you to actively pass data chunks into the parser, a file-like object passively responds to read() requests of the parser itself. Depending on the data source, either way may be more natural.

Note that the feed parser has its own error log called feed_error_log. Errors in the feed parser do not show up in the normal error_log and vice versa.

You can also combine the feed parser interface with the target parser:

>>> parser = etree.XMLParser(target = EchoTarget())

>>> parser.feed("<eleme")
>>> parser.feed("nt>some text</elem")
start element {}
data u'some text'
>>> parser.feed("ent>")
end element

>>> result = parser.close()
close
>>> print(result)
closed!

Again, this prevents the automatic creation of an XML tree and leaves all the event handling to the target object. The close() method of the parser forwards the return value of the target's close() method.

iterparse and iterwalk

As known from ElementTree, the iterparse() utility function returns an iterator that generates parser events for an XML file (or file-like object), while building the tree. The values are tuples (event-type, object). The event types supported by ElementTree and lxml.etree are the strings 'start', 'end', 'start-ns' and 'end-ns'.

The 'start' and 'end' events represent opening and closing elements. They are accompanied by the respective Element instance. By default, only 'end' events are generated:

>>> xml = '''\
... <root>
...   <element key='value'>text</element>
...   <element>text</element>tail
...   <empty-element xmlns="http://testns/" />
... </root>
... '''

>>> context = etree.iterparse(StringIO(xml))
>>> for action, elem in context:
...     print("%s: %s" % (action, elem.tag))
end: element
end: element
end: {http://testns/}empty-element
end: root

The resulting tree is available through the root property of the iterator:

>>> context.root.tag
'root'

The other event types can be activated with the events keyword argument:

>>> events = ("start", "end")
>>> context = etree.iterparse(StringIO(xml), events=events)
>>> for action, elem in context:
...     print("%s: %s" % (action, elem.tag))
start: root
start: element
end: element
start: element
end: element
start: {http://testns/}empty-element
end: {http://testns/}empty-element
end: root

The 'start-ns' and 'end-ns' events notify about namespace declarations. They do not come with Elements. Instead, the value of the 'start-ns' event is a tuple (prefix, namespaceURI) that designates the beginning of a prefix-namespace mapping. The corresponding end-ns event does not have a value (None). It is common practice to use a list as namespace stack and pop the last entry on the 'end-ns' event.

>>> print(xml[:-1])
<root>
  <element key='value'>text</element>
  <element>text</element>tail
  <empty-element xmlns="http://testns/" />
</root>

>>> events = ("start", "end", "start-ns", "end-ns")
>>> context = etree.iterparse(StringIO(xml), events=events)
>>> for action, elem in context:
...     if action in ('start', 'end'):
...         print("%s: %s" % (action, elem.tag))
...     elif action == 'start-ns':
...         print("%s: %s" % (action, elem))
...     else:
...         print(action)
start: root
start: element
end: element
start: element
end: element
start-ns: ('', 'http://testns/')
start: {http://testns/}empty-element
end: {http://testns/}empty-element
end-ns
end: root

Selective tag events

As an extension over ElementTree, lxml.etree accepts a tag keyword argument just like element.iter(tag). This restricts events to a specific tag or namespace:

>>> context = etree.iterparse(StringIO(xml), tag="element")
>>> for action, elem in context:
...     print("%s: %s" % (action, elem.tag))
end: element
end: element

>>> events = ("start", "end")
>>> context = etree.iterparse(
...             StringIO(xml), events=events, tag="{http://testns/}*")
>>> for action, elem in context:
...     print("%s: %s" % (action, elem.tag))
start: {http://testns/}empty-element
end: {http://testns/}empty-element

Comments and PIs

As an extension over ElementTree, the iterparse() function in lxml.etree also supports the event types 'comment' and 'pi' for the respective XML structures.

>>> commented_xml = '''\
... <?some pi ?>
... <!-- a comment -->
... <root>
...   <element key='value'>text</element>
...   <!-- another comment -->
...   <element>text</element>tail
...   <empty-element xmlns="http://testns/" />
... </root>
... '''

>>> events = ("start", "end", "comment", "pi")
>>> context = etree.iterparse(StringIO(commented_xml), events=events)
>>> for action, elem in context:
...     if action in ('start', 'end'):
...         print("%s: %s" % (action, elem.tag))
...     elif action == 'pi':
...         print("%s: -%s=%s-" % (action, elem.target, elem.text))
...     else: # 'comment'
...         print("%s: -%s-" % (action, elem.text))
pi: -some=pi -
comment: - a comment -
start: root
start: element
end: element
comment: - another comment -
start: element
end: element
start: {http://testns/}empty-element
end: {http://testns/}empty-element
end: root

>>> print(context.root.tag)
root

Modifying the tree

You can modify the element and its descendants when handling the 'end' event. To save memory, for example, you can remove subtrees that are no longer needed:

>>> context = etree.iterparse(StringIO(xml))
>>> for action, elem in context:
...     print(len(elem))
...     elem.clear()
0
0
0
3
>>> context.root.getchildren()
[]

WARNING: During the 'start' event, the descendants and following siblings are not yet available and should not be accessed. During the 'end' event, the element and its descendants can be freely modified, but its following siblings should not be accessed. During either of the two events, you must not modify or move the ancestors (parents) of the current element. You should also avoid moving or discarding the element itself. The golden rule is: do not touch anything that will have to be touched again by the parser later on.

If you have elements with a long list of children in your XML file and want to save more memory during parsing, you can clean up the preceding siblings of the current element:

>>> for event, element in etree.iterparse(StringIO(xml)):
...     # ... do something with the element
...     element.clear()                 # clean up children
...     while element.getprevious() is not None:
...         del element.getparent()[0]  # clean up preceding siblings

The while loop deletes multiple siblings in a row. This is only necessary if you skipped over some of them using the tag keyword argument. Otherwise, a simple if should do. The more selective your tag is, however, the more thought you will have to put into finding the right way to clean up the elements that were skipped. Therefore, it is sometimes easier to traverse all elements and do the tag selection by hand in the event handler code.

iterwalk

A second extension over ElementTree is the iterwalk() function. It behaves exactly like iterparse(), but works on Elements and ElementTrees:

>>> root = etree.XML(xml)
>>> context = etree.iterwalk(
...             root, events=("start", "end"), tag="element")
>>> for action, elem in context:
...     print("%s: %s" % (action, elem.tag))
start: element
end: element
start: element
end: element

>>> f = StringIO(xml)
>>> context = etree.iterparse(
...             f, events=("start", "end"), tag="element")

>>> for action, elem in context:
...     print("%s: %s" % (action, elem.tag))
start: element
end: element
start: element
end: element

Python unicode strings

lxml.etree has broader support for Python unicode strings than the ElementTree library. First of all, where ElementTree would raise an exception, the parsers in lxml.etree can handle unicode strings straight away. This is most helpful for XML snippets embedded in source code using the XML() function:

>>> uxml = u'<test> \uf8d1 + \uf8d2 </test>'
>>> uxml
u'<test> \uf8d1 + \uf8d2 </test>'
>>> root = etree.XML(uxml)

This requires, however, that unicode strings do not specify a conflicting encoding themselves and thus lie about their real encoding:

>>> etree.XML( u'<?xml version="1.0" encoding="ASCII"?>\n' + uxml )
Traceback (most recent call last):
  ...
ValueError: Unicode strings with encoding declaration are not supported.

Similarly, you will get errors when you try the same with HTML data in a unicode string that specifies a charset in a meta tag of the header. You should generally avoid converting XML/HTML data to unicode before passing it into the parsers. It is both slower and error prone.

Serialising to Unicode strings

To serialize the result, you would normally use the tostring() module function, which serializes to plain ASCII by default or a number of other byte encodings if asked for:

>>> etree.tostring(root)
b'<test> &#63697; + &#63698; </test>'

>>> etree.tostring(root, encoding='UTF-8', xml_declaration=False)
b'<test> \xef\xa3\x91 + \xef\xa3\x92 </test>'

As an extension, lxml.etree recognises the unicode type as an argument to the encoding parameter to build a Python unicode representation of a tree:

>>> etree.tostring(root, encoding=unicode)
u'<test> \uf8d1 + \uf8d2 </test>'

>>> el = etree.Element("test")
>>> etree.tostring(el, encoding=unicode)
u'<test/>'

>>> subel = etree.SubElement(el, "subtest")
>>> etree.tostring(el, encoding=unicode)
u'<test><subtest/></test>'

>>> tree = etree.ElementTree(el)
>>> etree.tostring(tree, encoding=unicode)
u'<test><subtest/></test>'

The result of tostring(encoding=unicode) can be treated like any other Python unicode string and then passed back into the parsers. However, if you want to save the result to a file or pass it over the network, you should use write() or tostring() with a byte encoding (typically UTF-8) to serialize the XML. The main reason is that unicode strings returned by tostring(encoding=unicode) are not byte streams and they never have an XML declaration to specify their encoding. These strings are most likely not parsable by other XML libraries.

For normal byte encodings, the tostring() function automatically adds a declaration as needed that reflects the encoding of the returned string. This makes it possible for other parsers to correctly parse the XML byte stream. Note that using tostring() with UTF-8 is also considerably faster in most cases.

Generated on: 2010-09-02.