I'm trying to write a validation script that will validate XML against the NITF DTD, http://www.iptc.org/std/NITF/3.4/specification/dtd/nitf-3-4.dtd. Based on this post I came up with the following simple script to validate a NITF XML document. Bellow is the error message I get when the script is run, which isn't very descriptive and makes it hard to debug. Any help is appreciated.
#!/usr/bin/env python
def main():
from lxml import etree, objectify
from StringIO import StringIO
f = open('nitf_test.xml')
xml_doc = f.read()
f.close()
f = open('nitf-3-4.dtd')
dtd_doc = f.read()
f.close()
dtd = etree.DTD(StringIO(dtd_doc))
tree = objectify.parse(StringIO(xml_doc))
dtd.validate(tree)
if __name__ == '__main__':
main()
Traceback error message:
Traceback (most recent call last):
File "./test_nitf_doc.py", line 23, in <module>
main()
File "./test_nitf_doc.py", line 16, in main
dtd = etree.DTD(StringIO(dtd_doc))
File "dtd.pxi", line 43, in lxml.etree.DTD.__init__ (src/lxml/lxml.etree.c:126056)
File "dtd.pxi", line 117, in lxml.etree._parseDtdFromFilelike (src/lxml/lxml.etree.c:126727)
lxml.etree.DTDParseError: error parsing DTD
If I change the line:
dtd = etree.DTD(StringIO(dtd_doc))
To:
dtd = etree.DTD(dtd_doc)
The error I get is:
lxml.etree.DTDParseError: failed to load external entity "NULL"
I took a look at the nitf-3-4.dtd and found that it references an external module xhtml-ruby-1.mod which can be downloaded at this link. This needs to be present in the current directory so the DTD parser can load it.
Full working example (assuming you have a valid NITF document handy):
% wget http://www.iptc.org/std/NITF/3.4/specification/dtd/nitf-3-4.dtd
% wget http://www.iptc.org/std/NITF/3.4/specification/dtd/xhtml-ruby-1.mod
Python code:
from lxml import etree, objectify
dtd = etree.DTD(open('nitf-3-4.dtd', 'rb'))
tree = objectify.parse(open('nitf_test.xml', 'rb'))
print dtd.validate(tree)
Output:
% python nitf_test.py
True