递归函数中的 Python 生成器

Question

我试图在递归语句中使用生成器，但没有得到预期的结果。

一些背景知识:我正在使用语法分析树，概念上的目标是递归树直到我识别出专有名词(由“NNP”标记表示)，然后我尝试使用生成器来识别专有名词所在的每个名词短语(由“NP”表示)。

alist = ['ROOT', ['S', ['NP', ['PRP', 'We']], ['VP', ['VBP', 'have'], ['VP', ['VBN', 'received'], ['NP', ['NN', 'information']],
        ['PP', ['IN', 'from'], ['NP', ['NP', ['DT', 'a'], ['NN', 'source']], ['VP', ['VBN', 'entitled'], ['PP', ['TO', 'to'],
        ['NP', ['NN', 'belief']]], [',', ','], ['SBAR', ['IN', 'that'], ['S', ['NP', ['NNP', 'Lincoln']], ['VP', ['VP', ['VBZ', 'has'],
        ['VP', ['VBN', 'paid'], ['NP', ['DT', 'a'], ['JJ', 'hurried'], ['NN', 'visit']], ['PP', ['TO', 'to'],
        ['NP', ['NP', ['DT', 'the'], ['NNP', 'Army']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'Potomac']]]]],
        [',', ','], ['PRN', ['-LRB-', '-LRB-'], ['ADVP', ['RB', 'now']], ['ADJP', ['JJ', 'burrowing'], ['PP', ['IN', 'on'],
        ['NP', ['NP', ['DT', 'the'], ['NN', 'north'], ['NN', 'bank']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'James']]],
        [',', ',']]]], ['-RRB-', '-RRB-']]]], ['CC', 'and'], ['VP', ['VBD', 'satisfied'], ['NP', ['PRP', 'himself']],
        [',', ','], ['PP', ['IN', 'by'], ['NP', ['JJ', 'personal'], ['NN', 'observation']]], [',', ','],
        ['PP', ['IN', 'in'], ['NP', ['NN', 'regard']]], ['PP', ['TO', 'to'], ['NP', ['NP', ['DT', 'the'], ['JJ', 'true'], ['NN', 'situation']],
        ['PP', ['IN', 'of'], ['NP', ['NNS', 'affairs']]]]]]]]]]]]]], ['.', '.']]]

def PullNP(NNP, NPLists):
    if NNP in NPLists:
        print "Pulling relevant NP"
        print NNP
        yield NNP
    for thing in NPLists:
        if NNP in thing:
            PullNP(thing, NPLists)
        else:
            for s in thing:
                if str(type(s)) == "<type 'list'>" and NNP in s: PullNP(s, NPLists)


def RecurseNNP(alist, pastlists=None, count=None):
    if pastlists is None: pastlists = []
    if count is None: count = 0
    if 'NNP' in alist[0]:
        NNPs = PullNP(alist, pastlists)
        print NNPs
        for np in NNPs:
            print np
    else:
        if str(type(alist)) == "<type 'list'>":
            if alist[0] == 'NP':
                pastlists.append(alist)
            for x in alist[1:]:
                RecurseNNP(x, pastlists, count)

RecurseNNP(alist)

如果我运行这段代码，我会得到这样的输出:

<generator object PullNP at 0x0288B648>
<generator object PullNP at 0x02885558>
<generator object PullNP at 0x02885558>
<generator object PullNP at 0x02885558>

遍历生成器对象不会产生任何输出。但是，如果我删除 yield 语句并仅将 PullNP 作为递归函数运行，我可以确认打印语句包含我希望它们输出的内容。即我希望我的生成器包含这些列表:

Pulling relevant NP
['NP', ['NNP', 'Lincoln']]
Pulling relevant NP
['NP', ['DT', 'the'], ['NNP', 'Army']]
Pulling relevant NP
['NP', ['NP', ['DT', 'the'], ['NNP', 'Army']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'Potomac']]]]
Pulling relevant NP
['NP', ['DT', 'the'], ['NNP', 'Army']]
Pulling relevant NP
['NP', ['NP', ['DT', 'the'], ['NNP', 'Army']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'Potomac']]]]
Pulling relevant NP
['NP', ['DT', 'the'], ['NNP', 'Potomac']]
Pulling relevant NP
['NP', ['NP', ['DT', 'the'], ['NNP', 'Army']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'Potomac']]]]
Pulling relevant NP
['NP', ['DT', 'the'], ['NNP', 'James']]
Pulling relevant NP
['NP', ['NP', ['DT', 'the'], ['NN', 'north'], ['NN', 'bank']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'James']]], [',', ',']]

我已经阅读了解释生成器和 yield 的主要堆栈溢出帖子，但我仍然不明白为什么我的生成器没有输出任何东西。

Answer 1

仅仅调用子生成器不会使生成器(迭代器)运行。您需要迭代返回值(或对其显式调用 next)以运行并使用产生的值。

这是递归函数。因此，将检索到的项目返回给调用者。注意标有# <---的代码部分

alist = ['ROOT', ['S', ['NP', ['PRP', 'We']], ['VP', ['VBP', 'have'], ['VP', ['VBN', 'received'], ['NP', ['NN', 'information']],
        ['PP', ['IN', 'from'], ['NP', ['NP', ['DT', 'a'], ['NN', 'source']], ['VP', ['VBN', 'entitled'], ['PP', ['TO', 'to'],
        ['NP', ['NN', 'belief']]], [',', ','], ['SBAR', ['IN', 'that'], ['S', ['NP', ['NNP', 'Lincoln']], ['VP', ['VP', ['VBZ', 'has'],
        ['VP', ['VBN', 'paid'], ['NP', ['DT', 'a'], ['JJ', 'hurried'], ['NN', 'visit']], ['PP', ['TO', 'to'],
        ['NP', ['NP', ['DT', 'the'], ['NNP', 'Army']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'Potomac']]]]],
        [',', ','], ['PRN', ['-LRB-', '-LRB-'], ['ADVP', ['RB', 'now']], ['ADJP', ['JJ', 'burrowing'], ['PP', ['IN', 'on'],
        ['NP', ['NP', ['DT', 'the'], ['NN', 'north'], ['NN', 'bank']], ['PP', ['IN', 'of'], ['NP', ['DT', 'the'], ['NNP', 'James']]],
        [',', ',']]]], ['-RRB-', '-RRB-']]]], ['CC', 'and'], ['VP', ['VBD', 'satisfied'], ['NP', ['PRP', 'himself']],
        [',', ','], ['PP', ['IN', 'by'], ['NP', ['JJ', 'personal'], ['NN', 'observation']]], [',', ','],
        ['PP', ['IN', 'in'], ['NP', ['NN', 'regard']]], ['PP', ['TO', 'to'], ['NP', ['NP', ['DT', 'the'], ['JJ', 'true'], ['NN', 'situation']],
        ['PP', ['IN', 'of'], ['NP', ['NNS', 'affairs']]]]]]]]]]]]]], ['.', '.']]]

def PullNP(NNP, NPLists):
    if NNP in NPLists:
        print "Pulling relevant NP"
        print NNP
        yield NNP
    for thing in NPLists:
        if NNP in thing:
            for nnp in PullNP(thing, NPLists):
                yield nnp
        else:
            for s in thing:
                if isinstance(s, list) and NNP in s:
                    for nnp in PullNP(s, NPLists):  # <---
                        yield nnp                   # <---


def RecurseNNP(alist, pastlists=None, count=None):
    if pastlists is None: pastlists = []
    if count is None: count = 0
    if 'NNP' in alist[0]:
        NNPs = PullNP(alist, pastlists)
        print NNPs
        for np in NNPs:
            print np
    else:
        if str(type(alist)) == "<type 'list'>":
            if alist[0] == 'NP':
                pastlists.append(alist)
            for x in alist[1:]:
                RecurseNNP(x, pastlists, count)

RecurseNNP(alist)