19.7.1.1. 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 - ElementTree represents the whole XML document as a tree,and Element represents a single node in this tree. Interactions with the whole document (reading and writing to/from files) are usually done on the ElementTree level. Interactions with a single XML element and its sub-elements are done on the Element level.

19.7.1.2. 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)

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

As an 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'

19.7.1.3. Finding interesting elements

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, 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'}

Element.findall() finds only elements with a tag which are direct children of the current element. Element.find() finds the first child with a particular tag,and Element.text accesses the element’s text content. 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 XPath.

19.7.1.4. Modifying an XML File

ElementTree provides a simple way to build XML documents and write them to files. The ElementTree.write() method serves this purpose.

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

19.7.1.5. Building XML documents

The 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>

19.7.1.6. Parsing XML with Namespaces

If the XML input has namespaces,tags and attributes with prefixes in the form prefix:soMetag get expanded to {uri}soMetag where the prefix is replaced by the full URI. Also,if there is a default namespace,that full URI gets prepended to all of the non-prefixed tags.

Here is an XML example that incorporates two namespaces,one with the prefix “fictional” and the other serving as the default namespace:

<?xml version="1.0"?>
<actors xmlns:fictional="http://characters.example.com"
        xmlns="http://people.example.com">
    <actor>
        <name>John Cleese</name>
        <fictional:character>Lancelot</fictional:character>
        <fictional:character>Archie Leach</fictional:character>
    </actor>
    <actor>
        <name>Eric Idle</name>
        <fictional:character>Sir Robin</fictional:character>
        <fictional:character>Gunther</fictional:character>
        <fictional:character>Commander Clement</fictional:character>
    </actor>
</actors>

One way to search and explore this XML example is to manually add the URI to every tag or attribute in the xpath of a find() or findall():

root = fromstring(xml_text)
for actor in root.findall('{http://people.example.com}actor'):
    name = actor.find('{http://people.example.com}name')
    print name.text
    for char in actor.findall('{http://characters.example.com}character'):
        print ' |-->', char.text

A better way to search the namespaced XML example is to create a dictionary with your own prefixes and use those in the search functions:

ns = {'real_person': 'http://people.example.com',
      'role': 'http://characters.example.com'}

for actor in root.findall('real_person:actor', ns):
    name = actor.find('real_person:name', ns)
    print name.text
    for char in actor.findall('role:character', ns):
        print ' |-->', char.text

These two approaches both output:

John Cleese
 |--> Lancelot
 |--> Archie Leach
Eric Idle
 |--> Sir Robin
 |--> Gunther
 |--> Commander Clement

19.7.1.7. Additional resources

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

19.7.2.1. 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 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]")

19.7.2.2. Supported XPath Syntax

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.

19.7.3.1. Functions

xml.etree.ElementTree.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.

xml.etree.ElementTree.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.

xml.etree.ElementTree.fromstring(text)

Parses an XML section from a string constant. Same as XML(). text is a string containing XML data. Returns an Element instance.

xml.etree.ElementTree.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 XMLParser parser is used. Returns an Element instance.

New in version 2.7.

xml.etree.ElementTree.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.

xml.etree.ElementTree.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 XMLParser parser is used. parser is not supported by cElementTree. Returns an iterator providing (event, elem) pairs.

Note

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.

xml.etree.ElementTree.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 XMLParser parser is used. Returns an ElementTree instance.

xml.etree.ElementTree.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.

xml.etree.ElementTree.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.

New in version 2.7.

xml.etree.ElementTree.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.

xml.etree.ElementTree.tostring(element,encoding="us-ascii",method="xml")

Generates a string representation of an XML element,including all subelements. element is an 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.

xml.etree.ElementTree.tostringlist(element,including all subelements. element is an 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).

New in version 2.7.

xml.etree.ElementTree.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 XMLParser parser is used. Returns an Element instance.

xml.etree.ElementTree.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 XMLParser parser is used. Returns a tuple containing an Element instance and a dictionary.

19.7.3.2. Element Objects

class xml.etree.ElementTree.Element(tag,**extra)

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

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

tag

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

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.

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.

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.

clear()

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

get(key,default=None)

Gets the element attribute named key.

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

items()

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

keys()

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

set(key,value)

Set the attribute key on the element to value.

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

append(subelement)

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

extend(subelements)

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

New in version 2.7.

find(match)

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

findall(match)

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

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.

getchildren()

Deprecated since version 2.7: Use list(elem) or iteration.

getiterator(tag=None)

Deprecated since version 2.7: Use method Element.iter() instead.

insert(index,element)

Inserts a subelement at the given position in this element.

iter(tag=None)

Creates a tree 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.

New in version 2.7.

iterfind(match)

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

New in version 2.7.

itertext()

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

New in version 2.7.

makeelement(tag,attrib)

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

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.

Element objects also support the following sequence type methods for working with subelements: __delitem__(), __getitem__(), __setitem__(), __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"

19.7.3.3. ElementTree Objects

class xml.etree.ElementTree.ElementTree(element=None,file=None)

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.

_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.

find(match)

Same as Element.find(),starting at the root of the tree.

findall(match)

Same as Element.findall(),starting at the root of the tree.

findtext(match,default=None)

Same as Element.findtext(),starting at the root of the tree.

getiterator(tag=None)

Deprecated since version 2.7: Use method ElementTree.iter() instead.

getroot()

Returns the root element for this tree.

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)

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.

New in version 2.7.

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.

write(file,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")

19.7.3.4. QName Objects

class xml.etree.ElementTree.QName(text_or_uri,tag=None)

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. QName instances are opaque.

19.7.3.5. TreeBuilder Objects

class xml.etree.ElementTree.TreeBuilder(element_factory=None)

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 Element instances when given.

close()

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

data(data)

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

end(tag)

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

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 TreeBuilder object can provide the following 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 TreeBuilder class.

New in version 2.7.

19.7.3.6. XMLParser Objects

class xml.etree.ElementTree.XMLParser(html=0,target=None,encoding=None)

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.

close()

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

doctype(name,system)

Deprecated since version 2.7: Define the TreeBuilder.doctype() method on a custom TreeBuilder target.

Feed(data)

Feeds data to the parser. data is encoded data.

XMLParser.feed() calls target‘s start() method for each opening tag,its end() method for each closing tag,and data is processed by method data(). XMLParser.close() calls target‘s method close(). 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