==============================
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::
..
1 Parsers
1.1 Parser options
1.2 Parsing HTML
1.3 Doctype information
2 The feed parser interface
3 iterparse and iterwalk
3.1 Selective tag events
3.2 Modifying the tree
3.3 iterwalk
4 Python unicode strings
4.1 Serialising to Unicode strings
The usual setup procedure::
>>> from lxml import etree
>>> from StringIO import StringIO
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)
>>> print etree.tostring(root)
<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))
>>> print etree.tostring(tree.getroot())
<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)
>>> print etree.tostring(tree.getroot())
<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
* 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
* 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):
...
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, error.column
1 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 something usable without raising an exception. 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)
>>> print etree.tostring(tree.getroot(), pretty_print=True),
<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)
>>> print etree.tostring(html, pretty_print=True),
<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.
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`: http://effbot.org/elementtree/elementtree-xmlparser.htm
`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", tag, attrib
... def end(self, tag):
... print "end", tag
... def data(self, data):
... print "data", repr(data)
... def close(self):
... print "close"
... return "closed!"
>>> parser = etree.XMLParser(target = EchoTarget())
>>> result = etree.XML("<element>some text</element>", parser)
start element {}
data u'some text'
end element
close
>>> print result
closed!
Note that the parser does *not* build a tree in this case. The result
of the parser run is what 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.
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. Note that you can only use one interface at a
time with each parser: the ``parse()`` or ``XML()`` functions, or the feed
parser interface.
.. _`ElementTree parsers`: http://effbot.org/elementtree/elementtree-xmlparser.htm
To start parsing with a feed parser, just call its ``feed()`` method::
>>> 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
usages will block till the end of times. So make sure you also close it 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 creating 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 are 'start', 'end', 'start-ns' and 'end-ns'.
The 'start' and 'end' events represent opening and closing elements and are
accompanied by the respective element. By default, only 'end' events are
generated::
>>> xml = '''\
... <root>
... <element key='value'>text</element>
... <element>text</element>tail
... <empty-element xmlns="testns" />
... </root>
... '''
>>> context = etree.iterparse(StringIO(xml))
>>> for action, elem in context:
... print action, elem.tag
end element
end element
end {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 action, elem.tag
start root
start element
end element
start element
end element
start {testns}empty-element
end {testns}empty-element
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 action, elem.tag
end element
end element
>>> events = ("start", "end")
>>> context = etree.iterparse(
... StringIO(xml), events=events, tag="{testns}*")
>>> for action, elem in context:
... print action, elem.tag
start {testns}empty-element
end {testns}empty-element
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.
The 'start-ns' and 'end-ns' events notify about namespace declarations and
generate tuples ``(prefix, URI)``::
>>> events = ("start-ns", "end-ns")
>>> context = etree.iterparse(StringIO(xml), events=events)
>>> for action, obj in context:
... print action, obj
start-ns ('', 'testns')
end-ns None
It is common practice to use a list as namespace stack and pop the last entry
on the 'end-ns' event.
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 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 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)
'<test>  +  </test>'
>>> etree.tostring(root, encoding='UTF-8', xml_declaration=False)
'<test> \xef\xa3\x91 + \xef\xa3\x92 </test>'
As an extension, lxml.etree recognises the unicode type as encoding 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.
