How to navigate a nltk.tree.Tree?

问题

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?

回答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)

回答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))

回答3:

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

回答4:

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')]

来源：https://stackoverflow.com/questions/14841997/how-to-navigate-a-nltk-tree-tree

标签

tree

nltk