XML/XHTML/HTML differences, bugs... and howto

Andrew Robinson andrew3 at r3dsolutions.com
Wed Jan 23 16:22:23 CET 2013


Good day :),

I've been exploring XML parsers in python; particularly: 
xml.etree.cElementTree; and I'm trying to figure out how to do it 
incrementally, for very large XML files -- although I don't think the 
problems are restricted to incremental parsing.

First problem:
I've come across an issue where etree silently drops text without 
telling me; and separate.

I am under the impression that XHTML is a subset of XML (eg:defined 
tags), and that once an HTML file is converted to XHTML, the body of the 
document can be handled entirely as XML.

If I convert a (partial/contrived) html file like:

<html>
     <div>
         <p> This is example <b>bold</b> text.
     </div>
</html>

to XHTML, I might do --right or wrong-- (1):

<html>
     <div>
         <p /> This is example <b>bold</b> text.
     </div>
</html>

or, alternate difference: (2): "<p> This is example <b>bold</b> text. </p>"

But, when I parse with etree,  in example (1) both "This is an example" 
and "text." are dropped;
The missing text is part of the start, or end event tags, in the 
incrementally parsed method.

Likewise: In example (2), only "text" gets dropped.

So, etree is silently dropping all text following a close tag, but 
before another open tag happens.

Q:
Isn't XML supposed to error out when invalid xml is parsed?
Is there a way in etree to recover/access the dropped text?
If not -- is the a python library issue, or the underlying expat.so, 
etc. library.

Secondly;
I have an XML file which will grow larger than memory on a target 
machine, so here's what I want to do:

Given a source XML file, and a destination file:
1) iteratively scan part of the source tree.
2) Optionally Modify some of scanned tree.
3) Write partial scan/tree out to the destination file.
4) Free memory of no-longer needed (partial) source XML.
5) continue scanning a new section of the source file... eg: goto step 1 
until source file is exhausted.

But, I don't see a way to write portions of an XML tree, or iteratively 
write a tree to disk.
How can this be done?

:)  Thanks!






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