In Python ElementTree how can I get list of all ancestors of an element in tree?

Question

I need "get_ancestors_recursively" function.
A sample run can be

>>> dump(tr)
<anc1>
  <anc2>
    <element> </element>
  </anc2>
</anc1>
>>> input_element = tr.getiterator("element")[0]
>>> get_ancestors_recursively(input_element)
['anc1', 'anc2']

Can somebody help me with this ?

Answer 1

In the latest version of ElementTree (v1.3 or later), you can simply do

input_element.find('..')

recursively. However, the ElementTree that ships with Python doesn't have this functionality, and I don't see anything in the Element class that looks upwards.

I believe this means you have to do it the hard way: via an exhaustive search of the element tree.

def get_ancestors_recursively(e, b):
    "Finds ancestors of b in the element tree e."
    return _get_ancestors_recursively(e.getroot(), b, [])

def _get_ancestors_recursively(s, b, acc):
    "Recursive variant. acc is the built-up list of ancestors so far."
    if s == b:
        return acc
    else:
        for child in s.getchildren():
            newacc = acc[:]
            newacc.append(s)
            res = _get_ancestors_recursively(child, b, newacc)
            if res is not None:
                return res
        return None

This is slow because of the DFS, and cranks out a lot of lists for garbage collection, but if you can deal with that it should be fine.