- lxml

- Developing with lxml

- Extending lxml

- Developing lxml

# 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

- Parsers
  - Parser options
  - Error log
  - Parsing HTML
  - Doctype information
- The target parser interface
- The feed parser interface
- iterparse and iterwalk
  - Selective tag events
  - Comments and PIs
  - Modifying the tree
  - iterwalk
- Python unicode strings
  - Serialising to Unicode strings

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
- 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):
    ...
    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 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)

    >>> result = etree.tostring(tree.getroot(), pretty_print=True)
    >>> 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)
    >>> 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.

## 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!

Note that the parser does not build a tree in this case. 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="testns" />
    ... </root>
    ... '''

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