Using Python and lxml to validate XML against an external DTD

Question

I'm trying to validate an XML file against an external DTD referenced in the doctype tag. Specifically:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE en-export SYSTEM "http://xml.evernote.com/pub/evernote-export3.dtd">
...the rest of the document...

I'm using Python 3.3 and the lxml module. From reading http://lxml.de/validation.html#validation-at-parse-time, I've thrown this together:

enexFile = open(sys.argv[2], mode="rb") # sys.argv[2] is the path to an XML file in local storage.
enexParser = etree.XMLParser(dtd_validation=True)
enexTree = etree.parse(enexFile, enexParser)

From what I understand of validation.html, the lxml library should now take care of retrieving the DTD and performing validation. But instead, I get this:

$ ./mapwrangler.py validate notes.enex
Traceback (most recent call last):
  File "./mapwrangler.py", line 27, in <module>
    enexTree = etree.parse(enexFile, enexParser)
  File "lxml.etree.pyx", line 3239, in lxml.etree.parse (src/lxml/lxml.etree.c:69955)
  File "parser.pxi", line 1769, in lxml.etree._parseDocument (src/lxml/lxml.etree.c:102257)
  File "parser.pxi", line 1789, in lxml.etree._parseFilelikeDocument (src/lxml/lxml.etree.c:102516)
  File "parser.pxi", line 1684, in lxml.etree._parseDocFromFilelike (src/lxml/lxml.etree.c:101442)
  File "parser.pxi", line 1134, in lxml.etree._BaseParser._parseDocFromFilelike (src/lxml/lxml.etree.c:97069)
  File "parser.pxi", line 582, in lxml.etree._ParserContext._handleParseResultDoc (src/lxml/lxml.etree.c:91275)
  File "parser.pxi", line 683, in lxml.etree._handleParseResult (src/lxml/lxml.etree.c:92461)
  File "parser.pxi", line 622, in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:91757)
lxml.etree.XMLSyntaxError: Validation failed: no DTD found !, line 3, column 43

This surprises me, because if I turn off validation, then the document parses in just fine and I can do print(enexTree.docinfo.doctype) to get

$ ./mapwrangler.py validate notes.enex
<!DOCTYPE en-export SYSTEM "http://xml.evernote.com/pub/evernote-export3.dtd">

So it looks to me like there shouldn't be any problem finding the DTD.

Thanks for your help.

Answer 1

You need to add no_network=False when constructing the parser object. This option is set to True by default.

From the documentation of parser options at http://lxml.de/parsing.html#parsers:

no_network - prevent network access when looking up external documents (on by default)