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
The target parser interface
The feed parser interface
Incremental event parsing
iterparse and iterwalk
- iterwalk
Python unicode strings
- Serialising to Unicode strings

The usual setup procedure:

>>> from lxml import etree

The following examples also use StringIO or BytesIO to show how to parse from files and file-like objects. Both are available in the io module:

from io import StringIO, BytesIO

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, also known as ignorable whitespace. This is best used together with a DTD or schema (which tells data and noise apart), otherwise a heuristic will be applied.
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)
collect_ids - collect XML IDs in a hash table while parsing (on by default). Disabling this can substantially speed up parsing of documents with many different IDs if the hash lookup is not used afterwards.

Other keyword arguments:

encoding - override the document encoding
target - a parser target object that will receive the parse events (see The target parser interface)
schema - an XMLSchema to validate against (see validation)

Error log

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

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

>>> tree = etree.XML("<root>\n</b>", parser)  # doctest: +ELLIPSIS
Traceback (most recent call last):
  ...
lxml.etree.XMLSyntaxError: Opening and ending tag mismatch: root line 1 and b, line 2, column 5...

>>> 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)
2
>>> print(error.column)
5

Each entry in the log has the following properties:

message: the message text
domain: the domain ID (see the lxml.etree.ErrorDomains class)
type: the message type ID (see the lxml.etree.ErrorTypes class)
level: the log level ID (see the lxml.etree.ErrorLevels class)
line: the line at which the message originated (if applicable)
column: the character column at which the message originated (if applicable)
filename: the name of the file in which the message originated (if applicable)

For convenience, there are also three properties that provide readable names for the ID values:

domain_name
type_name
level_name

To filter for a specific kind of message, use the different filter_*() methods on the error log (see the lxml.etree._ListErrorLog class).

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.

Also note that the HTML parser is meant to parse HTML documents. For XHTML documents, use the XML parser, which is namespace aware.

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. Since lxml 3.5, the doctype references are mutable.

>>> 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

>>> docinfo.system_url = None
>>> docinfo.public_id = None
>>> print(etree.tostring(tree))
<!DOCTYPE html>
<html><body/></html>

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(object):
...     def start(self, tag, attrib):
...         print("start %s %r" % (tag, dict(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!

Starting with lxml 2.3, the .close() method will also be called in the error case. This diverges from the behaviour of ElementTree, but allows target objects to clean up their state in all situations, so that the parser can reuse them afterwards.

>>> class CollectorTarget(object):
...     def __init__(self):
...         self.events = []
...     def start(self, tag, attrib):
...         self.events.append("start %s %r" % (tag, dict(attrib)))
...     def end(self, tag):
...         self.events.append("end %s" % tag)
...     def data(self, data):
...         self.events.append("data %r" % data)
...     def comment(self, text):
...         self.events.append("comment %s" % text)
...     def close(self):
...         self.events.append("close")
...         return "closed!"

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

>>> result = etree.XML("<element>some</error>",
...                    parser)        # doctest: +ELLIPSIS
Traceback (most recent call last):
  ...
lxml.etree.XMLSyntaxError: Opening and ending tag mismatch...

>>> for event in parser.target.events:
...     print(event)
start element {}
data u'some'
close

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.

Incremental event parsing

In Python 3.4, the xml.etree.ElementTree package gained an extension to the feed parser interface that is implemented by the XMLPullParser class. It additionally allows processing parse events after each incremental parsing step, by calling the .read_events() method and iterating over the result. This is most useful for non-blocking execution environments where data chunks arrive one after the other and should be processed as far as possible in each step.

The same feature is available in lxml 3.3. The basic usage is as follows:

>>> parser = etree.XMLPullParser(events=('start', 'end'))

>>> def print_events(parser):
...     for action, element in parser.read_events():
...         print('%s: %s' % (action, element.tag))

>>> parser.feed('<root>some text')
>>> print_events(parser)
start: root
>>> print_events(parser)    # well, no more events, as before ...

>>> parser.feed('<child><a />')
>>> print_events(parser)
start: child
start: a
end: a

>>> parser.feed('</child></roo')
>>> print_events(parser)
end: child
>>> parser.feed('t>')
>>> print_events(parser)
end: root

Just like the normal feed parser, the XMLPullParser builds a tree in memory (and you should always call the .close() method when done with parsing):

>>> root = parser.close()
>>> etree.tostring(root)
b'<root>some text<child><a/></child></root>'

However, since the parser provides incremental access to that tree, you can explicitly delete content that you no longer need once you have processed it. Read the section on Modifying the tree below to see what you can do here and what kind of modifications you should avoid.

In lxml, it is enough to call the .read_events() method once as the iterator it returns can be reused when new events are available.

Also, as known from other iterators in lxml, you can pass a tag argument that selects which parse events are returned by the .read_events() iterator.

Event types

The parse events 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, whereas the example above requested the generation of both 'start' and 'end' events.

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.

>>> def print_events(events):
...     for action, obj in events:
...         if action in ('start', 'end'):
...             print("%s: %s" % (action, obj.tag))
...         elif action == 'start-ns':
...             print("%s: %s" % (action, obj))
...         else:
...             print(action)

>>> event_types = ("start", "end", "start-ns", "end-ns")
>>> parser = etree.XMLPullParser(event_types)
>>> events = parser.read_events()

>>> parser.feed('<root><element>')
>>> print_events(events)
start: root
start: element
>>> parser.feed('text</element><element>text</element>')
>>> print_events(events)
end: element
start: element
end: element
>>> parser.feed('<empty-element xmlns="http://testns/" />')
>>> print_events(events)
start-ns: ('', 'http://testns/')
start: {http://testns/}empty-element
end: {http://testns/}empty-element
end-ns
>>> parser.feed('</root>')
>>> print_events(events)
end: 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:

>>> parser = etree.XMLPullParser()
>>> events = parser.read_events()

>>> parser.feed('<root><element key="value">text</element>')
>>> parser.feed('<element><child /></element>')
>>> for action, elem in events:
...     print('%s: %d' % (elem.tag, len(elem)))  # processing
...     elem.clear()                             # delete children
element: 0
child: 0
element: 1
>>> parser.feed('<empty-element xmlns="http://testns/" /></root>')
>>> for action, elem in events:
...     print('%s: %d' % (elem.tag, len(elem)))  # processing
...     elem.clear()                             # delete children
{http://testns/}empty-element: 0
root: 3

>>> root = parser.close()
>>> etree.tostring(root)
b'<root/>'

WARNING: During the 'start' event, any content of the element, such as the descendants, following siblings or text, is not yet available and should not be accessed. Only attributes are guaranteed to be set. 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 parser.read_events():
...     # ... 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.

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:

>>> parser = etree.XMLPullParser(tag="element")

>>> parser.feed('<root><element key="value">text</element>')
>>> parser.feed('<element><child /></element>')
>>> parser.feed('<empty-element xmlns="http://testns/" /></root>')

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

>>> event_types = ("start", "end")
>>> parser = etree.XMLPullParser(event_types, tag="{http://testns/}*")

>>> parser.feed('<root><element key="value">text</element>')
>>> parser.feed('<element><child /></element>')
>>> parser.feed('<empty-element xmlns="http://testns/" /></root>')

>>> for action, elem in parser.read_events():
...     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 XMLPullParser in lxml.etree also supports the event types 'comment' and 'pi' for the respective XML structures.

>>> event_types = ("start", "end", "comment", "pi")
>>> parser = etree.XMLPullParser(event_types)

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

>>> for action, elem in parser.read_events():
...     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

>>> root = parser.close()
>>> print(root.tag)
root

Events with custom targets

You can combine the pull parser with a parser target. In that case, it is the target's responsibility to generate event values. Whatever it returns from its .start() and .end() methods will be returned by the pull parser as the second item of the parse events tuple.

>>> class Target(object):
...     def start(self, tag, attrib):
...         print('-> start(%s)' % tag)
...         return '>>START: %s<<' % tag
...     def end(self, tag):
...         print('-> end(%s)' % tag)
...         return '>>END: %s<<' % tag
...     def close(self):
...         print('-> close()')
...         return "CLOSED!"

>>> event_types = ('start', 'end')
>>> parser = etree.XMLPullParser(event_types, target=Target())

>>> parser.feed('<root><child1 /><child2 /></root>')
-> start(root)
-> start(child1)
-> end(child1)
-> start(child2)
-> end(child2)
-> end(root)

>>> for action, value in parser.read_events():
...     print('%s: %s' % (action, value))
start: >>START: root<<
start: >>START: child1<<
end: >>END: child1<<
start: >>START: child2<<
end: >>END: child2<<
end: >>END: root<<

>>> print(parser.close())
-> close()
CLOSED!

As you can see, the event values do not even have to be Element objects. The target is generally free to decide how it wants to create an XML tree or whatever else it wants to make of the parser callbacks. In many cases, however, you will want to make your custom target inherit from the TreeBuilder class in order to have it build a tree that you can process normally. The start() and .end() methods of TreeBuilder return the Element object that was created, so you can override them and modify the input or output according to your needs. Here is an example that filters attributes before they are being added to the tree:

>>> class AttributeFilter(etree.TreeBuilder):
...     def start(self, tag, attrib):
...         attrib = dict(attrib)
...         if 'evil' in attrib:
...             del attrib['evil']
...         return super(AttributeFilter, self).start(tag, attrib)

>>> parser = etree.XMLPullParser(target=AttributeFilter())
>>> parser.feed('<root><child1 test="123" /><child2 evil="YES" /></root>')

>>> for action, element in parser.read_events():
...     print('%s: %s(%r)' % (action, element.tag, element.attrib))
end: child1({'test': '123'})
end: child2({})
end: root({})

>>> root = parser.close()

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. You can think of it as a blocking wrapper around the XMLPullParser that automatically and incrementally reads data from the input file for you and provides a single iterator for them:

>>> 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

After parsing, 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

iterparse() also supports the tag argument for selective event iteration and several other parameters that control the parser setup. You can also use it to parse HTML input by passing html=True.

iterwalk

A second extension over ElementTree is the iterwalk() function. It behaves exactly like iterparse(), but works on Elements and ElementTrees. Here is an example for a tree parsed by iterparse():

>>> 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

>>> root = context.root

And now we can take the resulting in-memory tree and iterate over it using iterwalk() to get the exact same events without parsing the input again:

>>> 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

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:

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

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' +
...            u'<test> \uf8d1 + \uf8d2 </test>' )
Traceback (most recent call last):
  ...
ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.

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 name 'unicode' 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.