Xml comparison in Python

Question

Building on another SO question, how can one check whether two well-formed XML snippets are semantically equal. All I need is "equal" or not, since I'm using this for unit tests.

In the system I want, these would be equal (note the order of 'start' and 'end'):

<?xml version='1.0' encoding='utf-8' standalone='yes'?>
<Stats start="1275955200" end="1276041599">
</Stats>

# Reordered start and end

<?xml version='1.0' encoding='utf-8' standalone='yes'?>
<Stats end="1276041599" start="1275955200" >
</Stats>

I have lmxl and other tools at my disposal, and a simple function that only allows reordering of attributes would work fine as well!

---

Working snippet based on IanB's answer:

from formencode import xml_compare
# have to strip these or fromstring carps
xml1 = """    <?xml version='1.0' encoding='utf-8' standalone='yes'?>
    <Stats start="1275955200" end="1276041599"></Stats>"""
xml2 = """     <?xml version='1.0' encoding='utf-8' standalone='yes'?>
    <Stats start="1275955200" end="1276041599"></Stats>"""
xml3 = """ <?xml version='1.0' encoding='utf-8' standalone='yes'?>
    <Stats start="1275955200"></Stats>"""

from lxml import etree
tree1 = etree.fromstring(xml1.strip())
tree2 = etree.fromstring(xml2.strip())
tree3 = etree.fromstring(xml3.strip())

import sys
reporter = lambda x: sys.stdout.write(x + "\n")

assert xml_compare(tree1,tree2,reporter)
assert xml_compare(tree1,tree3,reporter) is False

Answer 1

If you take a DOM approach, you can traverse the two trees simultaneously while comparing nodes (node type, text, attributes) as you go.

A recursive solution will be the most elegant - just short-circuit further comparison once a pair of nodes are not "equal" or once you detect a leaf in one tree when it's a branch in another, etc.

Answer 2

You can use formencode.xml_doctest_compare -- the xml_compare function compares two ElementTree or lxml trees.