How to properly navigate an NLTK parse tree?

Question

NLTK is driving me nuts again.

How do I properly navigate through an NLTK tree (or ParentedTree)? I would like to identify a certain leaf with the parent node "VBZ", then I would like to move from there further up the tree and to the left to identify the NP node.

How do I do this? The NLTK tree class does not seem to be thought through... Or I am too stupid...

Thanks for your help!

Answer 1

Based on what you want to do, this should work. It will give you the closest left NP node first, then the second closest, etc. So, if you had a tree of (S (NP1) (VP (NP2) (VBZ))) , your np_trees list would have [ParentedTree(NP2), ParentedTree(NP1)] .

from nltk.tree import *

np_trees = []

def traverse(t):
    try:
        t.label()
    except AttributeError:
        return

    if t.label() == "VBZ":
        current = t
        while current.parent() is not None:

            while current.left_sibling() is not None:

                if current.left_sibling().label() == "NP":
                    np_trees.append(current.left_sibling())

                current = current.left_sibling()

            current = current.parent()

    for child in t:
        traverse(child)

tree = ParentedTree.fromstring("(S (NP (NNP)) (VP (VBZ) (NP (NNP))))")
traverse(tree)
print np_trees # [ParentedTree('NP', [ParentedTree('NNP', [])])]