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?

could you post the full grammar with the leaf nodes, then i can give you a clear example? — alvas, Feb 13 '13 at 07:41

score 19 · Answer 1 · answered Mar 08 '14 at 14:10

19

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)

answered Mar 08 '14 at 14:10

Melroy van den Berg

2,697
28
31

Another DFS example (with `ParentedTree`): http://stackoverflow.com/a/25972853/4115369 – Yibo Yang Nov 14 '16 at 07:17
Yea thanks. Be-aware that the interface of NLTK may change overtime. – Melroy van den Berg Dec 14 '16 at 12:46

Peter Enns · Answer 2 · 2013-03-04T23:18:04.133

9

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))

edited Mar 04 '13 at 23:18

answered Mar 04 '13 at 23:12

Peter Enns

600
5
8

3

with Python 3.5 and NLTK 3.2.2, the lambda function should use the label() property of node: filter=lambda x: x.label() == "NP" – Maciej Apr 18 '17 at 21:37

score 7 · Answer 3 · answered Dec 11 '14 at 03:25

here's a code sample for generating all the subtrees with a label 'NP'

def filt(x):
    return x.label()=='NP'

for subtree in t.subtrees(filter =  filt): # Generate all subtrees
    print subtree

for siblings, you might wanna take a look at the method ParentedTree.left_siblings()

for more details, here are some useful links.

http://www.nltk.org/howto/tree.html #some basic usage and example http://nbviewer.ipython.org/github/gmonce/nltk_parsing/blob/master/1.%20NLTK%20Syntax%20Trees.ipynb #a notebook playwith these methods

http://www.nltk.org/_modules/nltk/tree.html #all api with source

score 5 · Answer 4 · edited Feb 10 '20 at 18:44

Try this:

for a in tree:
        if type(a) is nltk.Tree:
            if a.node == 'NVN': # This climbs into your NVN tree
                for b in a:
                    if type(b) is nltk.Tree and b.node == 'NP':
                        print b.leaves() # This outputs your "NP"
                    else:
                        print b # This outputs your "VB.*"

It outputs this:

[('The_Pigs', 'NNS')]

('are', 'VBP')

[('a', 'DT'), ('Bristol-based', 'JJ'), ('punk', 'NN'), ('rock', 'NN'), ('band', 'NN')]

How to navigate a nltk.tree.Tree?

4 Answers4

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