python - 如何在python中将nltk树(斯坦福)转换为newick格式？

Question

我有这棵斯坦福树，我想将其转换为 newick 格式。

    (ROOT
     (S
        (NP (DT A) (NN friend))
        (VP
         (VBZ comes)
         (NP
           (NP (JJ early))
           (, ,)
           (NP
             (NP (NNS others))
             (SBAR
                (WHADVP (WRB when))
                (S (NP (PRP they)) (VP (VBP have) (NP (NN time))))))))))

Answer 1

可能有一些方法可以仅使用字符串处理来做到这一点，但我会解析它们并递归地以 newick 格式打印它们。一个最小的实现:

import re

class Tree(object):
    def __init__(self, label):
        self.label = label
        self.children = []

    @staticmethod
    def _tokenize(string):
        return list(reversed(re.findall(r'\(|\)|[^ \n\t()]+', string)))

    @classmethod
    def from_string(cls, string):
        tokens = cls._tokenize(string)
        return cls._tree(tokens)

    @classmethod
    def _tree(cls, tokens):
        t = tokens.pop()
        if t == '(':
            tree = cls(tokens.pop())
            for subtree in cls._trees(tokens):
                tree.children.append(subtree)
            return tree
        else:
            return cls(t)

    @classmethod
    def _trees(cls, tokens):
        while True:
            if not tokens:
                raise StopIteration
            if tokens[-1] == ')':
                tokens.pop()
                raise StopIteration
            yield cls._tree(tokens)

    def to_newick(self):
        if self.children and len(self.children) == 1:
            return ','.join(child.to_newick() for child in self.children)
        elif self.chilren:
            return '(' + ','.join(child.to_newick() for child in self.children) + ')'
        else:
            return self.label

请注意，当然，信息在转换过程中会丢失，因为只保留了终端节点。用法:

>>> s = """(ROOT (..."""
>>> Tree.from_string(s).to_newick()
...