parsing nested unbounded XML fields with ElementTree

Larry Martell larry.martell at gmail.com
Tue Nov 26 13:23:20 CET 2013


On Tue, Nov 26, 2013 at 2:38 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Larry.Martell... at 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?

Thanks!
-larry

>
> Alternatively, if you want to use lxml.etree instead of ElementTree, you
> can use it's iterwalk() function, which gives you the same thing but
> without recursion, as a plain iterator.
>
> http://lxml.de/parsing.html#iterparse-and-iterwalk



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