NLTK relation extraction returns nothing

Question

I am recently working on using nltk to extract relation from text. so i build a sample text:" Tom is the cofounder of Microsoft." and using following program to test and return nothing. I cannot figure out why.

I'm using NLTK version: 3.2.1, python version: 3.5.2.

Here is my code:

import re
import nltk
from nltk.sem.relextract import extract_rels, rtuple
from nltk.tokenize import sent_tokenize, word_tokenize


def test():
    with open('sample.txt', 'r') as f:
        sample = f.read()   # "Tom is the cofounder of Microsoft"

    sentences = sent_tokenize(sample)
    tokenized_sentences = [word_tokenize(sentence) for sentence in sentences]
    tagged_sentences = [nltk.tag.pos_tag(sentence) for sentence in tokenized_sentences]

    OF = re.compile(r'.*\bof\b.*')

    for i, sent in enumerate(tagged_sentences):
        sent = nltk.chunk.ne_chunk(sent) # ne_chunk method expects one tagged sentence
        rels = extract_rels('PER', 'GPE', sent, corpus='ace', pattern=OF, window=10) 
        for rel in rels:
            print('{0:<5}{1}'.format(i, rtuple(rel)))

if __name__ == '__main__':
    test()

---

1. After some debug, if found that when i changed the input as

"Gates was born in Seattle, Washington on October 28, 1955. "

the nltk.chunk.ne_chunk() output is:

(S (PERSON Gates/NNS) was/VBD born/VBN in/IN (GPE Seattle/NNP) ,/, (GPE Washington/NNP) on/IN October/NNP 28/CD ,/, 1955/CD ./.)

The test() returns:

[PER: 'Gates/NNS'] 'was/VBD born/VBN in/IN' [GPE: 'Seattle/NNP']

2. After i changed the input as:

"Gates was born in Seattle on October 28, 1955. "

The test() retuns nothing.

3. I digged into nltk/sem/relextract.py and find this strange

output is caused by function: semi_rel2reldict(pairs, window=5, trace=False), which returns result only when len(pairs) > 2, and that's why when one sentence with less than three NEs will return None.

Is this a bug or i used NLTK in wrong way?

Answer 1

Firstly, to chunk NEs with ne_chunk , the idiom would look something like this

>>> from nltk import ne_chunk, pos_tag, word_tokenize
>>> text = "Tom is the cofounder of Microsoft"
>>> chunked = ne_chunk(pos_tag(word_tokenize(text)))
>>> chunked
Tree('S', [Tree('PERSON', [('Tom', 'NNP')]), ('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN'), Tree('ORGANIZATION', [('Microsoft', 'NNP')])])

(see also https://stackoverflow.com/a/31838373/610569 )

Next let's look at the extract_rels function .

def extract_rels(subjclass, objclass, doc, corpus='ace', pattern=None, window=10):
    """
    Filter the output of ``semi_rel2reldict`` according to specified NE classes and a filler pattern.
    The parameters ``subjclass`` and ``objclass`` can be used to restrict the
    Named Entities to particular types (any of 'LOCATION', 'ORGANIZATION',
    'PERSON', 'DURATION', 'DATE', 'CARDINAL', 'PERCENT', 'MONEY', 'MEASURE').
    """

When you evoke this function:

extract_rels('PER', 'GPE', sent, corpus='ace', pattern=OF, window=10)

It performs 4 processes sequentially.

1. It checks whether your subjclass and objclass are valid

ie https://github.com/nltk/nltk/blob/develop/nltk/sem/relextract.py#L202 :

if subjclass and subjclass not in NE_CLASSES[corpus]:
    if _expand(subjclass) in NE_CLASSES[corpus]:
        subjclass = _expand(subjclass)
    else:
        raise ValueError("your value for the subject type has not been recognized: %s" % subjclass)
if objclass and objclass not in NE_CLASSES[corpus]:
    if _expand(objclass) in NE_CLASSES[corpus]:
        objclass = _expand(objclass)
    else:
        raise ValueError("your value for the object type has not been recognized: %s" % objclass)

2. It extracts "pairs" from your NE tagged inputs:

if corpus == 'ace' or corpus == 'conll2002':
    pairs = tree2semi_rel(doc)
elif corpus == 'ieer':
    pairs = tree2semi_rel(doc.text) + tree2semi_rel(doc.headline)
else:
    raise ValueError("corpus type not recognized")

Now let's see given your input sentence Tom is the cofounder of Microsoft , what does tree2semi_rel() returns:

>>> from nltk.sem.relextract import tree2semi_rel, semi_rel2reldict
>>> from nltk import word_tokenize, pos_tag, ne_chunk
>>> text = "Tom is the cofounder of Microsoft"
>>> chunked = ne_chunk(pos_tag(word_tokenize(text)))
>>> tree2semi_rel(chunked)
[[[], Tree('PERSON', [('Tom', 'NNP')])], [[('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN')], Tree('ORGANIZATION', [('Microsoft', 'NNP')])]]

So it returns a list of 2 lists, the first inner list consist of a blank list and the Tree that contains the "PERSON" tag.

[[], Tree('PERSON', [('Tom', 'NNP')])] 

The second list consist of the phrase is the cofounder of and the Tree that contains "ORGANIZATION".

Let's move on.

3. extract_rel then tries to change the pairs to some sort of relation dictionary

reldicts = semi_rel2reldict(pairs)

If we look what the semi_rel2reldict function returns with your example sentence, we see that this is where the empty list gets returns:

>>> tree2semi_rel(chunked)
[[[], Tree('PERSON', [('Tom', 'NNP')])], [[('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN')], Tree('ORGANIZATION', [('Microsoft', 'NNP')])]]
>>> semi_rel2reldict(tree2semi_rel(chunked))
[]

So let's look into the code of semi_rel2reldict https://github.com/nltk/nltk/blob/develop/nltk/sem/relextract.py#L144 :

def semi_rel2reldict(pairs, window=5, trace=False):
    """
    Converts the pairs generated by ``tree2semi_rel`` into a 'reldict': a dictionary which
    stores information about the subject and object NEs plus the filler between them.
    Additionally, a left and right context of length =< window are captured (within
    a given input sentence).
    :param pairs: a pair of list(str) and ``Tree``, as generated by
    :param window: a threshold for the number of items to include in the left and right context
    :type window: int
    :return: 'relation' dictionaries whose keys are 'lcon', 'subjclass', 'subjtext', 'subjsym', 'filler', objclass', objtext', 'objsym' and 'rcon'
    :rtype: list(defaultdict)
    """
    result = []
    while len(pairs) > 2:
        reldict = defaultdict(str)
        reldict['lcon'] = _join(pairs[0][0][-window:])
        reldict['subjclass'] = pairs[0][1].label()
        reldict['subjtext'] = _join(pairs[0][1].leaves())
        reldict['subjsym'] = list2sym(pairs[0][1].leaves())
        reldict['filler'] = _join(pairs[1][0])
        reldict['untagged_filler'] = _join(pairs[1][0], untag=True)
        reldict['objclass'] = pairs[1][1].label()
        reldict['objtext'] = _join(pairs[1][1].leaves())
        reldict['objsym'] = list2sym(pairs[1][1].leaves())
        reldict['rcon'] = _join(pairs[2][0][:window])
        if trace:
            print("(%s(%s, %s)" % (reldict['untagged_filler'], reldict['subjclass'], reldict['objclass']))
        result.append(reldict)
        pairs = pairs[1:]
    return result

The first thing that semi_rel2reldict() does is to check where there are more than 2 elements the output from tree2semi_rel() , which your example sentence doesn't:

>>> tree2semi_rel(chunked)
[[[], Tree('PERSON', [('Tom', 'NNP')])], [[('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN')], Tree('ORGANIZATION', [('Microsoft', 'NNP')])]]
>>> len(tree2semi_rel(chunked))
2
>>> len(tree2semi_rel(chunked)) > 2
False

Ah ha, that's why the extract_rel is returning nothing.

Now comes the question of how to make extract_rel() return something even with 2 elements from tree2semi_rel() ? Is that even possible?

Let's try a different sentence:

>>> text = "Tom is the cofounder of Microsoft and now he is the founder of Marcohard"
>>> chunked = ne_chunk(pos_tag(word_tokenize(text)))
>>> chunked
Tree('S', [Tree('PERSON', [('Tom', 'NNP')]), ('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN'), Tree('ORGANIZATION', [('Microsoft', 'NNP')]), ('and', 'CC'), ('now', 'RB'), ('he', 'PRP'), ('is', 'VBZ'), ('the', 'DT'), ('founder', 'NN'), ('of', 'IN'), Tree('PERSON', [('Marcohard', 'NNP')])])
>>> tree2semi_rel(chunked)
[[[], Tree('PERSON', [('Tom', 'NNP')])], [[('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN')], Tree('ORGANIZATION', [('Microsoft', 'NNP')])], [[('and', 'CC'), ('now', 'RB'), ('he', 'PRP'), ('is', 'VBZ'), ('the', 'DT'), ('founder', 'NN'), ('of', 'IN')], Tree('PERSON', [('Marcohard', 'NNP')])]]
>>> len(tree2semi_rel(chunked)) > 2
True
>>> semi_rel2reldict(tree2semi_rel(chunked))
[defaultdict(<type 'str'>, {'lcon': '', 'untagged_filler': 'is the cofounder of', 'filler': 'is/VBZ the/DT cofounder/NN of/IN', 'objsym': 'microsoft', 'objclass': 'ORGANIZATION', 'objtext': 'Microsoft/NNP', 'subjsym': 'tom', 'subjclass': 'PERSON', 'rcon': 'and/CC now/RB he/PRP is/VBZ the/DT', 'subjtext': 'Tom/NNP'})]

But that only confirms that extract_rel can't extract when tree2semi_rel returns pairs of < 2. What happens if we remove that condition of while len(pairs) > 2 ?

Why can't we do while len(pairs) > 1 ?

If we look closer into the code, we see the last line of populating the reldict, https://github.com/nltk/nltk/blob/develop/nltk/sem/relextract.py#L169 :

reldict['rcon'] = _join(pairs[2][0][:window])

It tries to access a 3rd element of the pairs and if the length of the pairs is 2, you'll get an IndexError .

So what happens if we remove that rcon key and simply change it to while len(pairs) >= 2 ?

To do that we have to override the semi_rel2redict() function:

>>> from nltk.sem.relextract import _join, list2sym
>>> from collections import defaultdict
>>> def semi_rel2reldict(pairs, window=5, trace=False):
...     """
...     Converts the pairs generated by ``tree2semi_rel`` into a 'reldict': a dictionary which
...     stores information about the subject and object NEs plus the filler between them.
...     Additionally, a left and right context of length =< window are captured (within
...     a given input sentence).
...     :param pairs: a pair of list(str) and ``Tree``, as generated by
...     :param window: a threshold for the number of items to include in the left and right context
...     :type window: int
...     :return: 'relation' dictionaries whose keys are 'lcon', 'subjclass', 'subjtext', 'subjsym', 'filler', objclass', objtext', 'objsym' and 'rcon'
...     :rtype: list(defaultdict)
...     """
...     result = []
...     while len(pairs) >= 2:
...         reldict = defaultdict(str)
...         reldict['lcon'] = _join(pairs[0][0][-window:])
...         reldict['subjclass'] = pairs[0][1].label()
...         reldict['subjtext'] = _join(pairs[0][1].leaves())
...         reldict['subjsym'] = list2sym(pairs[0][1].leaves())
...         reldict['filler'] = _join(pairs[1][0])
...         reldict['untagged_filler'] = _join(pairs[1][0], untag=True)
...         reldict['objclass'] = pairs[1][1].label()
...         reldict['objtext'] = _join(pairs[1][1].leaves())
...         reldict['objsym'] = list2sym(pairs[1][1].leaves())
...         reldict['rcon'] = []
...         if trace:
...             print("(%s(%s, %s)" % (reldict['untagged_filler'], reldict['subjclass'], reldict['objclass']))
...         result.append(reldict)
...         pairs = pairs[1:]
...     return result
... 
>>> text = "Tom is the cofounder of Microsoft"
>>> chunked = ne_chunk(pos_tag(word_tokenize(text)))
>>> tree2semi_rel(chunked)
[[[], Tree('PERSON', [('Tom', 'NNP')])], [[('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN')], Tree('ORGANIZATION', [('Microsoft', 'NNP')])]]
>>> semi_rel2reldict(tree2semi_rel(chunked))
[defaultdict(<type 'str'>, {'lcon': '', 'untagged_filler': 'is the cofounder of', 'filler': 'is/VBZ the/DT cofounder/NN of/IN', 'objsym': 'microsoft', 'objclass': 'ORGANIZATION', 'objtext': 'Microsoft/NNP', 'subjsym': 'tom', 'subjclass': 'PERSON', 'rcon': [], 'subjtext': 'Tom/NNP'})]

Ah! It works but there's still a 4th step in extract_rels() .

4. It performs a filter of the reldict given the regex you have provided to the pattern parameter, https://github.com/nltk/nltk/blob/develop/nltk/sem/relextract.py#L222 :

relfilter = lambda x: (x['subjclass'] == subjclass and
                       len(x['filler'].split()) <= window and
                       pattern.match(x['filler']) and
                       x['objclass'] == objclass)

Now let's try it with the hacked version of semi_rel2reldict :

>>> text = "Tom is the cofounder of Microsoft"
>>> chunked = ne_chunk(pos_tag(word_tokenize(text)))
>>> tree2semi_rel(chunked)
[[[], Tree('PERSON', [('Tom', 'NNP')])], [[('is', 'VBZ'), ('the', 'DT'), ('cofounder', 'NN'), ('of', 'IN')], Tree('ORGANIZATION', [('Microsoft', 'NNP')])]]
>>> semi_rel2reldict(tree2semi_rel(chunked))
[defaultdict(<type 'str'>, {'lcon': '', 'untagged_filler': 'is the cofounder of', 'filler': 'is/VBZ the/DT cofounder/NN of/IN', 'objsym': 'microsoft', 'objclass': 'ORGANIZATION', 'objtext': 'Microsoft/NNP', 'subjsym': 'tom', 'subjclass': 'PERSON', 'rcon': [], 'subjtext': 'Tom/NNP'})]
>>> 
>>> pattern = re.compile(r'.*\bof\b.*')
>>> reldicts = semi_rel2reldict(tree2semi_rel(chunked))
>>> relfilter = lambda x: (x['subjclass'] == subjclass and
...                            len(x['filler'].split()) <= window and
...                            pattern.match(x['filler']) and
...                            x['objclass'] == objclass)
>>> relfilter
<function <lambda> at 0x112e591b8>
>>> subjclass = 'PERSON'
>>> objclass = 'ORGANIZATION'
>>> window = 5
>>> list(filter(relfilter, reldicts))
[defaultdict(<type 'str'>, {'lcon': '', 'untagged_filler': 'is the cofounder of', 'filler': 'is/VBZ the/DT cofounder/NN of/IN', 'objsym': 'microsoft', 'objclass': 'ORGANIZATION', 'objtext': 'Microsoft/NNP', 'subjsym': 'tom', 'subjclass': 'PERSON', 'rcon': [], 'subjtext': 'Tom/NNP'})]

It works! Now let's see it in tuple form:

>>> from nltk.sem.relextract import rtuple
>>> rels = list(filter(relfilter, reldicts))
>>> for rel in rels:
...     print rtuple(rel)
... 
[PER: 'Tom/NNP'] 'is/VBZ the/DT cofounder/NN of/IN' [ORG: 'Microsoft/NNP']

Answer 2

alvas' solution works superbly well! Minor modification though: instead of writing

>>> for rel in rels:
...     print rtuple(rel)

please use

>>> for rel in rels:
...    print (rtuple(rel))

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