Using lmxl/etree to merge two xml trees

mergetree.py

from lxml import etree
from copy import deepcopy

"""
This is an attempt to merge to XML trees.  It works but it probably doesn't do what you want because XML elements are not only identified
by their tag.  Take a POM for example, it will have many <dependency> nodes that are only distinguished by the text on their daughter 
node 'GroupId'.  This code does not take that into account.  
"""

def treeMerge(a, b):
    """Merge two xml trees A and B, so that each recursively found leaf element of B is added to A.  If the element
    already exists in A, it is replaced with B's version.  Tree structure is created in A as required to reflect the
    position of the leaf element in B.

    Given <top><first><a/><b/></first></top> and  <top><first><c/></first></top>, a merge results in
    <top><first><a/><b/><c/></first></top> (order not guaranteed)
    """

    def inner(aparent, bparent):
        for bchild in bparent:
            achild = aparent.xpath('./' + bchild.tag)
            if not achild:
                aparent.append(bchild)
            elif bchild.getchildren():
                inner(achild[0], bchild)

    res = deepcopy(a)
    inner(res, b)
    return res

test.py

lass Test_TreeMerge(unittest.TestCase):
    def test_level_zero(self):
        a = etree.fromstring("""<top></top>""")
        b = etree.fromstring("""<top2></top2>""")
        #result = donkey.treeMerge(a, b)

        # should give us an error

    def test_level_one(self):
        a = etree.fromstring("""<top><a/><b/><d><d2/></d></top>""")
        b = etree.fromstring("""<top><c/></top>""")
        result = donkey.treeMerge(a, b)

        self.assertEqual(len(result.xpath("/top")[0]), 4)
        self.assertEqual(len(result.xpath("/top/d/d2")), 1)

    def test_no_duplicates(self):
        a = etree.fromstring("""<top><a/></top>""")
        b = etree.fromstring("""<top><a/></top>""")
        result = donkey.treeMerge(a, b)

        self.assertEqual(len(result.xpath("/top")[0]), 1)

    def test_level_two(self):
        a = etree.fromstring("""<top><a/><b><b1/></b></top>""")
        b = etree.fromstring("""<top><b><b2/></b></top>""")
        result = donkey.treeMerge(a, b)
        print(etree.tostring(result))
        self.assertEqual(len(result.xpath("/top/a")), 1)
        self.assertEqual(len(result.xpath("/top/b/b1")), 1)
        self.assertEqual(len(result.xpath("/top/b/b2")), 1)

    def test_level_three(self):
        a = etree.fromstring("""<top><a/><b><b1><b11/><b22/></b1></b></top>""")
        b = etree.fromstring("""<top><a/><b><b1><b12/><b22/></b1></b></top>""")
        result = donkey.treeMerge(a, b)
        print(etree.tostring(result))
        self.assertEqual(len(result.xpath("/top/a")), 1)
        self.assertEqual(len(result.xpath("/top/b/b1/b11")), 1)
        self.assertEqual(len(result.xpath("/top/b/b1/b12")), 1)
        self.assertEqual(len(result.xpath("/top/b/b1/b22")), 1)

    def test_replaces_same_named_child(self):
        a = etree.fromstring("""<top><b foo=1></top>""")
        b = etree.fromstring("""<top><b foo=2></top>""")
        result = donkey.treeMerge(a, b)
        print(etree.tostring(result))
        self.assertEqual(len(result.xpath("/top/b[@foo=1]")), 0)
        self.assertEqual(len(result.xpath("/top/b[@foo=2]")), 1)


    def deal_with_namespaces(self):
        pass