[lxml-dev] Pickling objectified trees

[Christian Zagrodnick]

Hi!

On 2007-02-25 15:06:00 +0100, Stefan Behnel <stefan_ml at behnel.de> said:
> Christian Zagrodnick wrote:
>> the other day I had to pickle objectified trees. I just thought to
>> share my findings.
>> 
>> You might consider just registering the reduce function in lxml itself.
> 
> Interesting. Sure, why not? Objectify is totally about data classes after all.
> 
> Applied to the trunk (with small changes).


I found a may-be-considered-a-bug. The script below shows that when 
pickling the <cp/> node, the processing instruction is ommited.

When trying to pickle the root tree, the error is raised.

The problem is, that we pickle the `xml` object and get it back with 
all its descendants. But after unpickling the whole tree is not as it 
was before. So that's actually a bug.

I guess the best would be to

a) always serialize the roottree on pickle and
b) remember which part of the tree actually was pickelt so on unpickle 
this exact object can be restored.

I got to play with this a bit before I can deliver some useful code, thogh.


Regards,
Christian


----------------
import pickle
import lxml.etree
import lxml.objectify

xml = lxml.objectify.fromstring('<cp/><?foo?>')

print pickle.dumps(xml)
print pickle.dumps(xml.getroottree())
--------------------
-----------------
clxml.objectify
fromstring
p0
(S'<cp/>'
p1
tp2
Rp3
.
Traceback (most recent call last):
  File "pi.py", line 8, in ?
    print pickle.dumps(xml.getroottree())
  File "/Users/zagy/development/python/lib/python2.4/pickle.py", line 
1386, in dumps
    Pickler(file, protocol, bin).dump(obj)
  File "/Users/zagy/development/python/lib/python2.4/pickle.py", line 
231, in dump
    self.save(obj)
  File "/Users/zagy/development/python/lib/python2.4/pickle.py", line 
313, in save
    rv = reduce(self.proto)
  File "/Users/zagy/development/python/lib/python2.4/copy_reg.py", line 
69, in _reduce_ex
    raise TypeError, "can't pickle %s objects" % base.__name__
TypeError: can't pickle _ElementTree objects
--------------------------------------


-- 
Christian Zagrodnick · cz at gocept.com
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http://gocept.com · tel +49 345 1229889 4 · fax +49 345 1229889 1
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