CODEWITHSUNDEEP
pythonrecursiongenerator
GrantD71
GrantD71

Reputation: 1875

Python generator within a recursive function

I'm attempting to use a generator within a recursive statement, and I'm not getting the results I'm expecting.

A little background: I'm working with syntax parse trees, and the conceptual goal is to recurse down the tree until I identify proper nouns (signified by the 'NNP' tag) and then I attempt to use a generator to identify every noun phrase (signified by the 'NP') that the proper noun is within.

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)

If I run this code I get this output:

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

And iterating through the generator objects yields no output. However, if I remove the yield statement and merely run PullNP as a recursive function, I can confirm that the print statements contain what I want them to output. i.e. I would like my generators to contain these lists:

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']]], [',', ',']]

I have read the major stack overflow posts explaining generators and yield, and I still don't understand why my generator is not outputting anything.

Upvotes: 2

Views: 157

Answers (1)

falsetru
falsetru

Reputation: 369034

Just calling sub-generator will not make the generator (iterator) run. You need to iterate the returned value (or explicitly call next on it) to run & consume yielded value.

This is recusive function. So yield the retrieved item back to caller. Note the code part marked with # <---

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)

Upvotes: 1

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