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Date: Wed, 27 Nov 2013 09:58:36 -0500
Subject: Re: parsing nested unbounded XML fields with ElementTree
From: Larry Martell <larry.martell@gmail.com>
To: Stefan Behnel <stefan_ml@behnel.de>
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On Tue, Nov 26, 2013 at 8:20 AM, Stefan Behnel <stefan_ml@behnel.de> wrote:
> Larry Martell, 26.11.2013 13:23:
>> On Tue, Nov 26, 2013 at 2:38 AM, Stefan Behnel wrote:
>>> Larry.Martell...@gmail.com, 25.11.2013 23:22:
>>>> I have an XML file that has an element called "Node". These can be nested to any depth and the depth of the nesting is not known to me. I need to parse the file and preserve the nesting. For exmaple, if the XML file had:
>>>>
>>>> <Node Name="A">
>>>>    <Node Name="B">
>>>>       <Node Name="C">
>>>>         <Node Name="D">
>>>>           <Node Name="E">
>>>>
>>>> When I'm parsing Node "E" I need to know I'm in A/B/C/D/E. Problem is I don't know how deep this can be. This is the code I have so far:
>>>>
>>>> nodes = []
>>>>
>>>> def parseChild(c):
>>>>     if c.tag == 'Node':
>>>>         if 'Name' in c.attrib:
>>>>             nodes.append(c.attrib['Name'])
>>>>         for c1 in c:
>>>>             parseChild(c1)
>>>>     else:
>>>>         for node in nodes:
>>>>             print node,
>>>>         print c.tag
>>>>
>>>> for parent in tree.getiterator():
>>>>     for child in parent:
>>>>         for x in child:
>>>>             parseChild(x)
>>>
>>> This seems hugely redundant. tree.getiterator() already returns a recursive
>>> iterable, and then, for each nodes in your document, you are running
>>> recursively over its entire subtree. Meaning that you'll visit each node as
>>> many times as its depth in the tree.
>>>
>>>
>>>> My problem is that I don't know when I'm done with a node and I should
>>>> remove a level of nesting. I would think this is a fairly common
>>>> situation, but I could not find any examples of parsing a file like
>>>> this. Perhaps I'm going about it completely wrong.
>>>
>>> Your recursive traversal function tells you when you're done. If you drop
>>> the getiterator() bit, reaching the end of parseChild() means that you're
>>> done with the element and start backing up. So you can simply pass down a
>>> list of element names that you append() at the beginning of the function
>>> and pop() at the end, i.e. a stack. That list will then always give you the
>>> current path from the root node.
>>
>> Thanks for the reply. How can I remove getiterator()? Then I won't be
>> traversing the nodes of the tree. I can't iterate over tree. I am also
>> unclear on where to do the pop(). I tried putting it just after the
>> recursive call to parseChild() and I tried putting as the very last
>> statement in parseChild() - neither one gave the desired result. Can
>> you show me in code what you mean?
>
> untested:
>
>   nodes = []
>
>   def process_subtree(c, path):
>       name = c.get('Name') if c.tag == 'Node' else None
>       if name:
>           path.append(name)
>           nodes.append('/'.join(path))
>
>       for c1 in c:
>           process_subtree(c1, path)
>
>       if name:
>           path.pop()
>
>   process_subtree(tree.getroot(), [])

Thanks! This was extremely helpful and I've use these concepts to
write script that successfully parses my file.