Can ElementTree be told to preserve the order of attributes?

Question

I've written a fairly simple filter in python using ElementTree to munge the contexts of some xml files. And it works, more or less.

But it reorders the attributes of various tags, and I'd like it to not do that.

Does anyone know a switch I can throw to make it keep them in specified order?

Context for this

I'm working with and on a particle physics tool that has a complex, but oddly limited configuration system based on xml files. Among the many things setup that way are the paths to various static data files. These paths are hardcoded into the existing xml and there are no facilities for setting or varying them based on environment variables, and in our local installation they are necessarily in a different place.

This isn't a disaster because the combined source- and build-control tool we're using allows us to shadow certain files with local copies. But even thought the data fields are static the xml isn't, so I've written a script for fixing the paths, but with the attribute rearrangement diffs between the local and master versions are harder to read than necessary.

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This is my first time taking ElementTree for a spin (and only my fifth or sixth python project) so maybe I'm just doing it wrong.

Abstracted for simplicity the code looks like this:

tree = elementtree.ElementTree.parse(inputfile)
i = tree.getiterator()
for e in i:
    e.text = filter(e.text)
tree.write(outputfile)

Reasonable or dumb?

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Related links:

• How can I get the order of an element attribute list using Python xml.sax?
• Preserve order of attributes when modifying with minidom

Answer 1

Nope. ElementTree uses a dictionary to store attribute values, so it's inherently unordered.

Even DOM doesn't guarantee you attribute ordering, and DOM exposes a lot more detail of the XML infoset than ElementTree does. (There are some DOMs that do offer it as a feature, but it's not standard.)

Can it be fixed? Maybe. Here's a stab at it that replaces the dictionary when parsing with an ordered one. (I used odict as linked from PEP 372).

from xml.etree import ElementTree
import odict

class OrderedXMLTreeBuilder(ElementTree.XMLTreeBuilder):
    def _start_list(self, tag, attrib_in):
        fixname = self._fixname
        tag = fixname(tag)
        attrib= odict.odict()
        if attrib_in:
            for i in range(0, len(attrib_in), 2):
                attrib[fixname(attrib_in[i])] = self._fixtext(attrib_in[i+1])
        return self._target.start(tag, attrib)

>>> xmlf= StringIO.StringIO('<a b="c" d="e" f="g" j="k" h="i"/>')

>>> tree= ElementTree.ElementTree()
>>> root= tree.parse(xmlf, OrderedXMLTreeBuilder())
>>> root.attrib
odict.odict([('b', 'c'), ('d', 'e'), ('f', 'g'), ('j', 'k'), ('h', 'i')])

Looks potentially promising.

>>> s= StringIO.StringIO()
>>> tree.write(s)
>>> s.getvalue()
'<a b="c" d="e" f="g" h="i" j="k" />'

Bah, the serialiser outputs them in canonical order.

This looks like the line to blame, in ElementTree._write:

            items.sort() # lexical order

Subclassing or monkey-patching that is going to be annoying as it's right in the middle of a big method.

Unless you did something nasty like subclass odict and hack items to return a special subclass of list that ignores calls to sort(). Nah, probably that's even worse and I should go to bed before I come up with anything more horrible than that.

Answer 2

Wrong question. Should be: "Where do I find a diff gadget that works sensibly with XML files?

Answer: Google is your friend. First result for search on "xml diff" => this. There are a few more possibles.

Answer 3

From section 3.1 of the XML recommendation:

Note that the order of attribute specifications in a start-tag or empty-element tag is not significant.

Any system that relies on the order of attributes in an XML element is going to break.