How to navigate a nltk.tree.Tree?

Question

I've chunked a sentence using:

grammar = '''                                                                                                                   NP:                                                                                                                            {<DT>*(<NN.*>|<JJ.*>)*<NN.*>}                                                                                             NVN:                                                                                                                          {<NP><VB.*><NP>}                                                                                                         ''' chunker = nltk.chunk.RegexpParser(grammar) tree = chunker.parse(tagged) print tree 

The result looks like:

(S   (NVN     (NP The_Pigs/NNS)     are/VBP     (NP a/DT Bristol-based/JJ punk/NN rock/NN band/NN))   that/WDT   formed/VBN   in/IN   1977/CD   ./.) 

But now I'm stuck trying to figure out how to navigate that. I want to be able to find the NVN subtree, and access the left-side noun phrase ("The_Pigs"), the verb ("are") and the right-side noun phrase ("a Bristol-based punk rock band"). How do I do that?

Answer 1

Try:

ROOT = 'ROOT' tree = ... def getNodes(parent):     for node in parent:         if type(node) is nltk.Tree:             if node.label() == ROOT:                 print "======== Sentence ========="                 print "Sentence:", " ".join(node.leaves())             else:                 print "Label:", node.label()                 print "Leaves:", node.leaves()              getNodes(node)         else:             print "Word:", node  getNodes(tree) 

Answer 2

You could, of course, write your own depth first search... but there is an easier (better) way. If you want every subtree rooted at NVM, use Tree's subtree method with the filter parameter defined.

>>> print t (S     (NVN         (NP The_Pigs/NNS)         are/VBP         (NP a/DT Bristol-based/JJ punk/NN rock/NN band/NN))     that/WDT     formed/VBN     in/IN     1977/CD     ./.) >>> for i in t.subtrees(filter=lambda x: x.node == 'NVN'): ...     print i ...  (NVN     (NP The_Pigs/NNS)     are/VBP     (NP a/DT Bristol-based/JJ punk/NN rock/NN band/NN))