XML tree and elements

XML is an inherently hierarchical data format, and the most natural way to represent it is with a tree. ET has two classes for this purpose - :class:`ElementTree` represents the whole XML document as a tree, and :class:`Element` represents a single node in this tree. Interactions with the whole document (reading and writing to/from files) are usually done on the :class:`ElementTree` level. Interactions with a single XML element and its sub-elements are done on the :class:`Element` level.

Parsing XML

We'll be using the following XML document as the sample data for this section:

<?xml version="1.0"?>
<data>
    <country name="Liechtenstein">
        <rank>1</rank>
        <year>2008</year>
        <gdppc>141100</gdppc>
        <neighbor name="Austria" direction="E"/>
        <neighbor name="Switzerland" direction="W"/>
    </country>
    <country name="Singapore">
        <rank>4</rank>
        <year>2011</year>
        <gdppc>59900</gdppc>
        <neighbor name="Malaysia" direction="N"/>
    </country>
    <country name="Panama">
        <rank>68</rank>
        <year>2011</year>
        <gdppc>13600</gdppc>
        <neighbor name="Costa Rica" direction="W"/>
        <neighbor name="Colombia" direction="E"/>
    </country>
</data>

We have a number of ways to import the data. Reading the file from disk:

import xml.etree.ElementTree as ET
tree = ET.parse('country_data.xml')
root = tree.getroot()

Reading the data from a string:

root = ET.fromstring(country_data_as_string)

:func:`fromstring` parses XML from a string directly into an :class:`Element`, which is the root element of the parsed tree. Other parsing functions may create an :class:`ElementTree`. Check the documentation to be sure.

As an :class:`Element`, root has a tag and a dictionary of attributes:

>>> root.tag
'data'
>>> root.attrib
{}

It also has children nodes over which we can iterate:

>>> for child in root:
...   print child.tag, child.attrib
...
country {'name': 'Liechtenstein'}
country {'name': 'Singapore'}
country {'name': 'Panama'}

Children are nested, and we can access specific child nodes by index:

>>> root[0][1].text
'2008'

Finding interesting elements

:class:`Element` has some useful methods that help iterate recursively over all the sub-tree below it (its children, their children, and so on). For example, :meth:`Element.iter`:

>>> for neighbor in root.iter('neighbor'):
...   print neighbor.attrib
...
{'name': 'Austria', 'direction': 'E'}
{'name': 'Switzerland', 'direction': 'W'}
{'name': 'Malaysia', 'direction': 'N'}
{'name': 'Costa Rica', 'direction': 'W'}
{'name': 'Colombia', 'direction': 'E'}

:meth:`Element.findall` finds only elements with a tag which are direct children of the current element. :meth:`Element.find` finds the first child with a particular tag, and :attr:`Element.text` accesses the element's text content. :meth:`Element.get` accesses the element's attributes:

>>> for country in root.findall('country'):
...   rank = country.find('rank').text
...   name = country.get('name')
...   print name, rank
...
Liechtenstein 1
Singapore 4
Panama 68

More sophisticated specification of which elements to look for is possible by using :ref:`XPath <elementtree-xpath>`.

Modifying an XML File

:class:`ElementTree` provides a simple way to build XML documents and write them to files. The :meth:`ElementTree.write` method serves this purpose.

Once created, an :class:`Element` object may be manipulated by directly changing its fields (such as :attr:`Element.text`), adding and modifying attributes (:meth:`Element.set` method), as well as adding new children (for example with :meth:`Element.append`).

Let's say we want to add one to each country's rank, and add an updated attribute to the rank element:

>>> for rank in root.iter('rank'):
...   new_rank = int(rank.text) + 1
...   rank.text = str(new_rank)
...   rank.set('updated', 'yes')
...
>>> tree.write('output.xml')

Our XML now looks like this:

<?xml version="1.0"?>
<data>
    <country name="Liechtenstein">
        <rank updated="yes">2</rank>
        <year>2008</year>
        <gdppc>141100</gdppc>
        <neighbor name="Austria" direction="E"/>
        <neighbor name="Switzerland" direction="W"/>
    </country>
    <country name="Singapore">
        <rank updated="yes">5</rank>
        <year>2011</year>
        <gdppc>59900</gdppc>
        <neighbor name="Malaysia" direction="N"/>
    </country>
    <country name="Panama">
        <rank updated="yes">69</rank>
        <year>2011</year>
        <gdppc>13600</gdppc>
        <neighbor name="Costa Rica" direction="W"/>
        <neighbor name="Colombia" direction="E"/>
    </country>
</data>

We can remove elements using :meth:`Element.remove`. Let's say we want to remove all countries with a rank higher than 50:

>>> for country in root.findall('country'):
...   rank = int(country.find('rank').text)
...   if rank > 50:
...     root.remove(country)
...
>>> tree.write('output.xml')

Our XML now looks like this:

<?xml version="1.0"?>
<data>
    <country name="Liechtenstein">
        <rank updated="yes">2</rank>
        <year>2008</year>
        <gdppc>141100</gdppc>
        <neighbor name="Austria" direction="E"/>
        <neighbor name="Switzerland" direction="W"/>
    </country>
    <country name="Singapore">
        <rank updated="yes">5</rank>
        <year>2011</year>
        <gdppc>59900</gdppc>
        <neighbor name="Malaysia" direction="N"/>
    </country>
</data>

Building XML documents

The :func:`SubElement` function also provides a convenient way to create new sub-elements for a given element:

>>> a = ET.Element('a')
>>> b = ET.SubElement(a, 'b')
>>> c = ET.SubElement(a, 'c')
>>> d = ET.SubElement(c, 'd')
>>> ET.dump(a)
<a><b /><c><d /></c></a>

Additional resources

See http://effbot.org/zone/element-index.htm for tutorials and links to other docs.

Example

Here's an example that demonstrates some of the XPath capabilities of the module. We'll be using the countrydata XML document from the :ref:`Parsing XML <elementtree-parsing-xml>` section:

import xml.etree.ElementTree as ET

root = ET.fromstring(countrydata)

# Top-level elements
root.findall(".")

# All 'neighbor' grand-children of 'country' children of the top-level
# elements
root.findall("./country/neighbor")

# Nodes with name='Singapore' that have a 'year' child
root.findall(".//year/..[@name='Singapore']")

# 'year' nodes that are children of nodes with name='Singapore'
root.findall(".//*[@name='Singapore']/year")

# All 'neighbor' nodes that are the second child of their parent
root.findall(".//neighbor[2]")

Supported XPath syntax

.. tabularcolumns:: |l|L|

Predicates (expressions within square brackets) must be preceded by a tag name, an asterisk, or another predicate. position predicates must be preceded by a tag name.

Functions

.. function:: Comment(text=None)

   Comment element factory.  This factory function creates a special element
   that will be serialized as an XML comment by the standard serializer.  The
   comment string can be either a bytestring or a Unicode string.  *text* is a
   string containing the comment string.  Returns an element instance
   representing a comment.

.. function:: dump(elem)

   Writes an element tree or element structure to sys.stdout.  This function
   should be used for debugging only.

   The exact output format is implementation dependent.  In this version, it's
   written as an ordinary XML file.

   *elem* is an element tree or an individual element.

.. function:: fromstring(text)

   Parses an XML section from a string constant.  Same as :func:`XML`.  *text*
   is a string containing XML data.  Returns an :class:`Element` instance.

.. function:: fromstringlist(sequence, parser=None)

   Parses an XML document from a sequence of string fragments.  *sequence* is a
   list or other sequence containing XML data fragments.  *parser* is an
   optional parser instance.  If not given, the standard :class:`XMLParser`
   parser is used.  Returns an :class:`Element` instance.

   .. versionadded:: 2.7

.. function:: iselement(element)

   Checks if an object appears to be a valid element object.  *element* is an
   element instance.  Returns a true value if this is an element object.

.. function:: iterparse(source, events=None, parser=None)

   Parses an XML section into an element tree incrementally, and reports what's
   going on to the user.  *source* is a filename or file object containing XML
   data.  *events* is a list of events to report back.  If omitted, only "end"
   events are reported.  *parser* is an optional parser instance.  If not
   given, the standard :class:`XMLParser` parser is used.  *parser* is not
   supported by ``cElementTree``. Returns an :term:`iterator` providing
   ``(event, elem)`` pairs.

   .. note::

      :func:`iterparse` only guarantees that it has seen the ">"
      character of a starting tag when it emits a "start" event, so the
      attributes are defined, but the contents of the text and tail attributes
      are undefined at that point.  The same applies to the element children;
      they may or may not be present.

      If you need a fully populated element, look for "end" events instead.

.. function:: parse(source, parser=None)

   Parses an XML section into an element tree.  *source* is a filename or file
   object containing XML data.  *parser* is an optional parser instance.  If
   not given, the standard :class:`XMLParser` parser is used.  Returns an
   :class:`ElementTree` instance.

.. function:: ProcessingInstruction(target, text=None)

   PI element factory.  This factory function creates a special element that
   will be serialized as an XML processing instruction.  *target* is a string
   containing the PI target.  *text* is a string containing the PI contents, if
   given.  Returns an element instance, representing a processing instruction.

.. function:: register_namespace(prefix, uri)

   Registers a namespace prefix.  The registry is global, and any existing
   mapping for either the given prefix or the namespace URI will be removed.
   *prefix* is a namespace prefix.  *uri* is a namespace uri.  Tags and
   attributes in this namespace will be serialized with the given prefix, if at
   all possible.

   .. versionadded:: 2.7

.. function:: SubElement(parent, tag, attrib={}, **extra)

   Subelement factory.  This function creates an element instance, and appends
   it to an existing element.

   The element name, attribute names, and attribute values can be either
   bytestrings or Unicode strings.  *parent* is the parent element.  *tag* is
   the subelement name.  *attrib* is an optional dictionary, containing element
   attributes.  *extra* contains additional attributes, given as keyword
   arguments.  Returns an element instance.

.. function:: tostring(element, encoding="us-ascii", method="xml")

   Generates a string representation of an XML element, including all
   subelements.  *element* is an :class:`Element` instance.  *encoding* [1]_ is
   the output encoding (default is US-ASCII).  *method* is either ``"xml"``,
   ``"html"`` or ``"text"`` (default is ``"xml"``).  Returns an encoded string
   containing the XML data.

.. function:: tostringlist(element, encoding="us-ascii", method="xml")

   Generates a string representation of an XML element, including all
   subelements.  *element* is an :class:`Element` instance.  *encoding* [1]_ is
   the output encoding (default is US-ASCII).   *method* is either ``"xml"``,
   ``"html"`` or ``"text"`` (default is ``"xml"``).  Returns a list of encoded
   strings containing the XML data.  It does not guarantee any specific
   sequence, except that ``"".join(tostringlist(element)) ==
   tostring(element)``.

   .. versionadded:: 2.7

.. function:: XML(text, parser=None)

   Parses an XML section from a string constant.  This function can be used to
   embed "XML literals" in Python code.  *text* is a string containing XML
   data.  *parser* is an optional parser instance.  If not given, the standard
   :class:`XMLParser` parser is used.  Returns an :class:`Element` instance.

.. function:: XMLID(text, parser=None)

   Parses an XML section from a string constant, and also returns a dictionary
   which maps from element id:s to elements.  *text* is a string containing XML
   data.  *parser* is an optional parser instance.  If not given, the standard
   :class:`XMLParser` parser is used.  Returns a tuple containing an
   :class:`Element` instance and a dictionary.

Element Objects

Element class. This class defines the Element interface, and provides a reference implementation of this interface.

The element name, attribute names, and attribute values can be either bytestrings or Unicode strings. tag is the element name. attrib is an optional dictionary, containing element attributes. extra contains additional attributes, given as keyword arguments.

.. attribute:: tag

   A string identifying what kind of data this element represents (the
   element type, in other words).

.. attribute:: text

   The *text* attribute can be used to hold additional data associated with
   the element.  As the name implies this attribute is usually a string but
   may be any application-specific object.  If the element is created from
   an XML file the attribute will contain any text found between the element
   tags.

.. attribute:: tail

   The *tail* attribute can be used to hold additional data associated with
   the element.  This attribute is usually a string but may be any
   application-specific object.  If the element is created from an XML file
   the attribute will contain any text found after the element's end tag and
   before the next tag.

.. attribute:: attrib

   A dictionary containing the element's attributes.  Note that while the
   *attrib* value is always a real mutable Python dictionary, an ElementTree
   implementation may choose to use another internal representation, and
   create the dictionary only if someone asks for it.  To take advantage of
   such implementations, use the dictionary methods below whenever possible.

The following dictionary-like methods work on the element attributes.

.. method:: clear()

   Resets an element.  This function removes all subelements, clears all
   attributes, and sets the text and tail attributes to None.

.. method:: get(key, default=None)

   Gets the element attribute named *key*.

   Returns the attribute value, or *default* if the attribute was not found.

.. method:: items()

   Returns the element attributes as a sequence of (name, value) pairs.  The
   attributes are returned in an arbitrary order.

.. method:: keys()

   Returns the elements attribute names as a list.  The names are returned
   in an arbitrary order.

.. method:: set(key, value)

   Set the attribute *key* on the element to *value*.

The following methods work on the element's children (subelements).

.. method:: append(subelement)

   Adds the element *subelement* to the end of this elements internal list
   of subelements.

.. method:: extend(subelements)

   Appends *subelements* from a sequence object with zero or more elements.
   Raises :exc:`AssertionError` if a subelement is not a valid object.

   .. versionadded:: 2.7

.. method:: find(match)

   Finds the first subelement matching *match*.  *match* may be a tag name
   or path.  Returns an element instance or ``None``.

.. method:: findall(match)

   Finds all matching subelements, by tag name or path.  Returns a list
   containing all matching elements in document order.

.. method:: findtext(match, default=None)

   Finds text for the first subelement matching *match*.  *match* may be
   a tag name or path.  Returns the text content of the first matching
   element, or *default* if no element was found.  Note that if the matching
   element has no text content an empty string is returned.

.. method:: getchildren()

   .. deprecated:: 2.7
      Use ``list(elem)`` or iteration.

.. method:: getiterator(tag=None)

   .. deprecated:: 2.7
      Use method :meth:`Element.iter` instead.

.. method:: insert(index, element)

   Inserts a subelement at the given position in this element.

.. method:: iter(tag=None)

   Creates a tree :term:`iterator` with the current element as the root.
   The iterator iterates over this element and all elements below it, in
   document (depth first) order.  If *tag* is not ``None`` or ``'*'``, only
   elements whose tag equals *tag* are returned from the iterator.  If the
   tree structure is modified during iteration, the result is undefined.

   .. versionadded:: 2.7

.. method:: iterfind(match)

   Finds all matching subelements, by tag name or path.  Returns an iterable
   yielding all matching elements in document order.

   .. versionadded:: 2.7

.. method:: itertext()

   Creates a text iterator.  The iterator loops over this element and all
   subelements, in document order, and returns all inner text.

   .. versionadded:: 2.7

.. method:: makeelement(tag, attrib)

   Creates a new element object of the same type as this element.  Do not
   call this method, use the :func:`SubElement` factory function instead.

.. method:: remove(subelement)

   Removes *subelement* from the element.  Unlike the find\* methods this
   method compares elements based on the instance identity, not on tag value
   or contents.

:class:`Element` objects also support the following sequence type methods for working with subelements: :meth:`~object.__delitem__`, :meth:`~object.__getitem__`, :meth:`~object.__setitem__`, :meth:`~object.__len__`.

Caution: Elements with no subelements will test as False. This behavior will change in future versions. Use specific len(elem) or elem is None test instead.

element = root.find('foo')

if not element:  # careful!
    print "element not found, or element has no subelements"

if element is None:
    print "element not found"

ElementTree Objects

ElementTree wrapper class. This class represents an entire element hierarchy, and adds some extra support for serialization to and from standard XML.

element is the root element. The tree is initialized with the contents of the XML file if given.

.. method:: _setroot(element)

   Replaces the root element for this tree.  This discards the current
   contents of the tree, and replaces it with the given element.  Use with
   care.  *element* is an element instance.

.. method:: find(match)

   Same as :meth:`Element.find`, starting at the root of the tree.

.. method:: findall(match)

   Same as :meth:`Element.findall`, starting at the root of the tree.

.. method:: findtext(match, default=None)

   Same as :meth:`Element.findtext`, starting at the root of the tree.

.. method:: getiterator(tag=None)

   .. deprecated:: 2.7
      Use method :meth:`ElementTree.iter` instead.

.. method:: getroot()

   Returns the root element for this tree.

.. method:: iter(tag=None)

   Creates and returns a tree iterator for the root element.  The iterator
   loops over all elements in this tree, in section order.  *tag* is the tag
   to look for (default is to return all elements)

.. method:: iterfind(match)

   Finds all matching subelements, by tag name or path.  Same as
   getroot().iterfind(match). Returns an iterable yielding all matching
   elements in document order.

   .. versionadded:: 2.7

.. method:: parse(source, parser=None)

   Loads an external XML section into this element tree.  *source* is a file
   name or file object.  *parser* is an optional parser instance.  If not
   given, the standard XMLParser parser is used.  Returns the section
   root element.

.. method:: write(file, encoding="us-ascii", xml_declaration=None, \
                  default_namespace=None, method="xml")

   Writes the element tree to a file, as XML.  *file* is a file name, or a
   file object opened for writing.  *encoding* [1]_ is the output encoding
   (default is US-ASCII).  *xml_declaration* controls if an XML declaration
   should be added to the file.  Use False for never, True for always, None
   for only if not US-ASCII or UTF-8 (default is None).  *default_namespace*
   sets the default XML namespace (for "xmlns").  *method* is either
   ``"xml"``, ``"html"`` or ``"text"`` (default is ``"xml"``).  Returns an
   encoded string.

This is the XML file that is going to be manipulated:

<html>
    <head>
        <title>Example page</title>
    </head>
    <body>
        <p>Moved to <a href="http://example.org/">example.org</a>
        or <a href="http://example.com/">example.com</a>.</p>
    </body>
</html>

Example of changing the attribute "target" of every link in first paragraph:

>>> from xml.etree.ElementTree import ElementTree
>>> tree = ElementTree()
>>> tree.parse("index.xhtml")
<Element 'html' at 0xb77e6fac>
>>> p = tree.find("body/p")     # Finds first occurrence of tag p in body
>>> p
<Element 'p' at 0xb77ec26c>
>>> links = list(p.iter("a"))   # Returns list of all links
>>> links
[<Element 'a' at 0xb77ec2ac>, <Element 'a' at 0xb77ec1cc>]
>>> for i in links:             # Iterates through all found links
...     i.attrib["target"] = "blank"
>>> tree.write("output.xhtml")

QName Objects

QName wrapper. This can be used to wrap a QName attribute value, in order to get proper namespace handling on output. text_or_uri is a string containing the QName value, in the form {uri}local, or, if the tag argument is given, the URI part of a QName. If tag is given, the first argument is interpreted as an URI, and this argument is interpreted as a local name. :class:`QName` instances are opaque.

TreeBuilder Objects

Generic element structure builder. This builder converts a sequence of start, data, and end method calls to a well-formed element structure. You can use this class to build an element structure using a custom XML parser, or a parser for some other XML-like format. The element_factory is called to create new :class:`Element` instances when given.

.. method:: close()

   Flushes the builder buffers, and returns the toplevel document
   element.  Returns an :class:`Element` instance.

.. method:: data(data)

   Adds text to the current element.  *data* is a string.  This should be
   either a bytestring, or a Unicode string.

.. method:: end(tag)

   Closes the current element.  *tag* is the element name.  Returns the
   closed element.

.. method:: start(tag, attrs)

   Opens a new element.  *tag* is the element name.  *attrs* is a dictionary
   containing element attributes.  Returns the opened element.

In addition, a custom :class:`TreeBuilder` object can provide the following method:

.. method:: doctype(name, pubid, system)

   Handles a doctype declaration.  *name* is the doctype name.  *pubid* is
   the public identifier.  *system* is the system identifier.  This method
   does not exist on the default :class:`TreeBuilder` class.

   .. versionadded:: 2.7

XMLParser Objects

:class:`Element` structure builder for XML source data, based on the expat parser. html are predefined HTML entities. This flag is not supported by the current implementation. target is the target object. If omitted, the builder uses an instance of the standard TreeBuilder class. encoding [1] is optional. If given, the value overrides the encoding specified in the XML file.

.. method:: close()

   Finishes feeding data to the parser.  Returns an element structure.

.. method:: doctype(name, pubid, system)

   .. deprecated:: 2.7
      Define the :meth:`TreeBuilder.doctype` method on a custom TreeBuilder
      target.

.. method:: feed(data)

   Feeds data to the parser.  *data* is encoded data.

:meth:`XMLParser.feed` calls target's :meth:`start` method for each opening tag, its :meth:`end` method for each closing tag, and data is processed by method :meth:`data`. :meth:`XMLParser.close` calls target's method :meth:`close`. :class:`XMLParser` can be used not only for building a tree structure. This is an example of counting the maximum depth of an XML file:

>>> from xml.etree.ElementTree import XMLParser
>>> class MaxDepth:                     # The target object of the parser
...     maxDepth = 0
...     depth = 0
...     def start(self, tag, attrib):   # Called for each opening tag.
...         self.depth += 1
...         if self.depth > self.maxDepth:
...             self.maxDepth = self.depth
...     def end(self, tag):             # Called for each closing tag.
...         self.depth -= 1
...     def data(self, data):
...         pass            # We do not need to do anything with data.
...     def close(self):    # Called when all data has been parsed.
...         return self.maxDepth
...
>>> target = MaxDepth()
>>> parser = XMLParser(target=target)
>>> exampleXml = """
... <a>
...   <b>
...   </b>
...   <b>
...     <c>
...       <d>
...       </d>
...     </c>
...   </b>
... </a>"""
>>> parser.feed(exampleXml)
>>> parser.close()
4

Footnotes