Python lxml - using the xml:lang attribute to retrieve an element

Question

I have some xml which has multiple elements with the same name, but each is in a different language, for example:

<Title xml:lang="FR" type="main">Les Tudors</Title>
<Title xml:lang="DE" type="main">Die Tudors</Title>
<Title xml:lang="IT" type="main">The Tudors</Title>

Normally, I'd retrieve an element using its attributes as follows:

titlex = info.find('.//xmlns:Title[@someattribute=attributevalue]', namespaces=nsmap)

If I try and do this with [@xml:lang="FR"] (for example), I get the traceback error:

  File "D:/Python code/RBM CRID, Title, Genre/CRID, Title, Genre, Age rating, Episode Number, Descriptions V1.py", line 29, in <module>
    titlex = info.find('.//xmlns:Title[@xml:lang=PL]', namespaces=nsmap) 

  File "lxml.etree.pyx", line 1457, in lxml.etree._Element.find (src\lxml\lxml.etree.c:51435)

  File "C:\Python34\lib\site-packages\lxml\_elementpath.py", line 282, in find
    it = iterfind(elem, path, namespaces)

  File "C:\Python34\lib\site-packages\lxml\_elementpath.py", line 272, in iterfind
    selector = _build_path_iterator(path, namespaces)

  File "C:\Python34\lib\site-packages\lxml\_elementpath.py", line 256, in _build_path_iterator
    selector.append(ops[token[0]](_next, token))

  File "C:\Python34\lib\site-packages\lxml\_elementpath.py", line 134, in prepare_predicate
    token = next()

  File "C:\Python34\lib\site-packages\lxml\_elementpath.py", line 80, in xpath_tokenizer
    raise SyntaxError("prefix %r not found in prefix map" % prefix) SyntaxError: prefix 'xml' not found in prefix map

I'm not surprised by this, but I'd like suggestions on how to get around the issue.

Thanks!

As requested, a cut-down but complete set of code (It works as expected if I remove the [bitsinsquarebrackets]):

import lxml
import codecs

file_name = (input('Enter the file name, excluding .xml extension: ') + '.xml')# User inputs file name
print('Parsing ' + file_name)


#----- Sets up import and namespace

from lxml import etree

parser = lxml.etree.XMLParser()


tree = lxml.etree.parse(file_name, parser)                                 # Name of file to test goes here
root = tree.getroot()

nsmap = {'xmlns': 'urn:tva:metadata:2012',
         'mpeg7': 'urn:tva:mpeg7:2008'}

#----- This code writes the output to a file

with codecs.open(file_name+'.log', mode='w', encoding='utf-8') as f:                        # Name the output file
    f.write(u'CRID|Title|Genre|Rating|Short Synopsis|Medium Synopsis|Long Synopsis\n')
    for info in root.xpath('//xmlns:ProgramInformation', namespaces=nsmap):
       titlex = info.find('.//xmlns:Title[xml:lang="PL"]', namespaces=nsmap)             # Retreve the title
       title = titlex.text if titlex != None else 'Missing'             # If there isn't a title, print an alternative word
       f.write(u'{}\n'.format(title))                     # Write all the retrieved values to the same line with bar seperators and a new line

Answer 1

The xml prefix in xml:lang does not need to be declared in an XML document, but if you want to use xml:lang in XPath lookups, you have to define a prefix mapping in the Python code.

The xml prefix is reserved (as opposed to "normal" namespace prefixes which are arbitrary) and defined to be bound to http://www.w3.org/XML/1998/namespace. See the Namespaces in XML 1.0 W3C recommendation.

Example:

from lxml import etree

# Required mapping
nsmap = {"xml": "http://www.w3.org/XML/1998/namespace"}

XML = """
<root>
  <Title xml:lang="FR" type="main">Les Tudors</Title>
  <Title xml:lang="DE" type="main">Die Tudors</Title>
  <Title xml:lang="IT" type="main">The Tudors</Title>
</root>"""

doc = etree.fromstring(XML)

title_FR = doc.find('Title[@xml:lang="FR"]', namespaces=nsmap)
print title_FR.text

Output:

Les Tudors

---

If there is no mapping for the xml prefix, you get the "prefix 'xml' not found in prefix map" error. If the URI mapped to the xml prefix is not http://www.w3.org/XML/1998/namespace, the find method in the code snippet above does not return anything.

Answer 2

If you have control over the xml file , you should change the xml:lang attribute to lang .

Or if you do not have that control , I would suggest adding xml in the nsmap, like -

nsmap = {'xmlns': 'urn:tva:metadata:2012',
         'mpeg7': 'urn:tva:mpeg7:2008',
         'xml': '<namespace>'}