问题
I wanted to use wordnet lemmatizer in python and I have learnt that the default pos tag is NOUN and that it does not output the correct lemma for a verb, unless the pos tag is explicitly specified as VERB.
My question is what is the best shot inorder to perform the above lemmatization accurately?
I did the pos tagging using nltk.pos_tag and I am lost in integrating the tree bank pos tags to wordnet compatible pos tags. Please help
from nltk.stem.wordnet import WordNetLemmatizer
lmtzr = WordNetLemmatizer()
tagged = nltk.pos_tag(tokens)
I get the output tags in NN,JJ,VB,RB. How do I change these to wordnet compatible tags?
Also do I have to train nltk.pos_tag() with a tagged corpus or can I use it directly on my data to evaluate?
回答1:
First of all, you can use nltk.pos_tag() directly without training it.
The function will load a pretrained tagger from a file. You can see the file name
with nltk.tag._POS_TAGGER:
nltk.tag._POS_TAGGER
>>> 'taggers/maxent_treebank_pos_tagger/english.pickle'
As it was trained with the Treebank corpus, it also uses the Treebank tag set.
The following function would map the treebank tags to WordNet part of speech names:
from nltk.corpus import wordnet
def get_wordnet_pos(treebank_tag):
if treebank_tag.startswith('J'):
return wordnet.ADJ
elif treebank_tag.startswith('V'):
return wordnet.VERB
elif treebank_tag.startswith('N'):
return wordnet.NOUN
elif treebank_tag.startswith('R'):
return wordnet.ADV
else:
return ''
You can then use the return value with the lemmatizer:
from nltk.stem.wordnet import WordNetLemmatizer
lemmatizer = WordNetLemmatizer()
lemmatizer.lemmatize('going', wordnet.VERB)
>>> 'go'
Check the return value before passing it to the Lemmatizer because an empty string would give a KeyError.
回答2:
As in the source code of nltk.corpus.reader.wordnet (http://www.nltk.org/_modules/nltk/corpus/reader/wordnet.html)
#{ Part-of-speech constants
ADJ, ADJ_SAT, ADV, NOUN, VERB = 'a', 's', 'r', 'n', 'v'
#}
POS_LIST = [NOUN, VERB, ADJ, ADV]
回答3:
Steps to convert : Document->Sentences->Tokens->POS->Lemmas
import nltk
from nltk.stem import WordNetLemmatizer
from nltk.corpus import wordnet
#example text text = 'What can I say about this place. The staff of these restaurants is nice and the eggplant is not bad'
class Splitter(object):
"""
split the document into sentences and tokenize each sentence
"""
def __init__(self):
self.splitter = nltk.data.load('tokenizers/punkt/english.pickle')
self.tokenizer = nltk.tokenize.TreebankWordTokenizer()
def split(self,text):
"""
out : ['What', 'can', 'I', 'say', 'about', 'this', 'place', '.']
"""
# split into single sentence
sentences = self.splitter.tokenize(text)
# tokenization in each sentences
tokens = [self.tokenizer.tokenize(sent) for sent in sentences]
return tokens
class LemmatizationWithPOSTagger(object):
def __init__(self):
pass
def get_wordnet_pos(self,treebank_tag):
"""
return WORDNET POS compliance to WORDENT lemmatization (a,n,r,v)
"""
if treebank_tag.startswith('J'):
return wordnet.ADJ
elif treebank_tag.startswith('V'):
return wordnet.VERB
elif treebank_tag.startswith('N'):
return wordnet.NOUN
elif treebank_tag.startswith('R'):
return wordnet.ADV
else:
# As default pos in lemmatization is Noun
return wordnet.NOUN
def pos_tag(self,tokens):
# find the pos tagginf for each tokens [('What', 'WP'), ('can', 'MD'), ('I', 'PRP') ....
pos_tokens = [nltk.pos_tag(token) for token in tokens]
# lemmatization using pos tagg
# convert into feature set of [('What', 'What', ['WP']), ('can', 'can', ['MD']), ... ie [original WORD, Lemmatized word, POS tag]
pos_tokens = [ [(word, lemmatizer.lemmatize(word,self.get_wordnet_pos(pos_tag)), [pos_tag]) for (word,pos_tag) in pos] for pos in pos_tokens]
return pos_tokens
lemmatizer = WordNetLemmatizer()
splitter = Splitter()
lemmatization_using_pos_tagger = LemmatizationWithPOSTagger()
#step 1 split document into sentence followed by tokenization
tokens = splitter.split(text)
#step 2 lemmatization using pos tagger
lemma_pos_token = lemmatization_using_pos_tagger.pos_tag(tokens)
print(lemma_pos_token)
回答4:
You can create a map using the python default dict and take advantage of the fact that for the lemmatizer the default tag is Noun.
from nltk.corpus import wordnet as wn
from nltk.stem.wordnet import WordNetLemmatizer
from nltk import word_tokenize, pos_tag
from collections import defaultdict
tag_map = defaultdict(lambda : wn.NOUN)
tag_map['J'] = wn.ADJ
tag_map['V'] = wn.VERB
tag_map['R'] = wn.ADV
text = "Another way of achieving this task"
tokens = word_tokenize(text)
lmtzr = WordNetLemmatizer()
for token, tag in pos_tag(tokens):
lemma = lmtzr.lemmatize(token, tag_map[tag[0]])
print(token, "=>", lemma)
回答5:
@Suzana_K was working. But I there are some case result in KeyError as @ Clock Slave mention.
Convert treebank tags to Wordnet tag
from nltk.corpus import wordnet
def get_wordnet_pos(treebank_tag):
if treebank_tag.startswith('J'):
return wordnet.ADJ
elif treebank_tag.startswith('V'):
return wordnet.VERB
elif treebank_tag.startswith('N'):
return wordnet.NOUN
elif treebank_tag.startswith('R'):
return wordnet.ADV
else:
return None # for easy if-statement
Now, we only input pos into lemmatize function only if we have wordnet tag
from nltk.stem.wordnet import WordNetLemmatizer
lemmatizer = WordNetLemmatizer()
tagged = nltk.pos_tag(tokens)
for word, tag in tagged:
wntag = get_wordnet_pos(tag)
if wntag is None:# not supply tag in case of None
lemma = lemmatizer.lemmatize(word)
else:
lemma = lemmatizer.lemmatize(word, pos=wntag)
回答6:
You can do this in one line:
wnpos = lambda e: ('a' if e[0].lower() == 'j' else e[0].lower()) if e[0].lower() in ['n', 'r', 'v'] else 'n'
Then use wnpos(nltk_pos) to get the POS to give to .lemmatize(). In your case, lmtzr.lemmatize(word=tagged[0][0], pos=wnpos(tagged[0][1])).
回答7:
After searching from internet, I've found this solution: from sentence to "bag of words" derived after splitting, pos_tagging, lemmatizing and cleaning (from punctuation and "stopping words") operations. Here's my code:
from nltk.corpus import wordnet as wn
from nltk.wsd import lesk
from nltk.stem import WordNetLemmatizer
from nltk.corpus import stopwords
from nltk.tokenize import word_tokenize
punctuation = u",.?!()-_\"\'\\\n\r\t;:+*<>@#§^$%&|/"
stop_words_eng = set(stopwords.words('english'))
lemmatizer = WordNetLemmatizer()
tag_dict = {"J": wn.ADJ,
"N": wn.NOUN,
"V": wn.VERB,
"R": wn.ADV}
def extract_wnpostag_from_postag(tag):
#take the first letter of the tag
#the second parameter is an "optional" in case of missing key in the dictionary
return tag_dict.get(tag[0].upper(), None)
def lemmatize_tupla_word_postag(tupla):
"""
giving a tupla of the form (wordString, posTagString) like ('guitar', 'NN'), return the lemmatized word
"""
tag = extract_wnpostag_from_postag(tupla[1])
return lemmatizer.lemmatize(tupla[0], tag) if tag is not None else tupla[0]
def bag_of_words(sentence, stop_words=None):
if stop_words is None:
stop_words = stop_words_eng
original_words = word_tokenize(sentence)
tagged_words = nltk.pos_tag(original_words) #returns a list of tuples: (word, tagString) like ('And', 'CC')
original_words = None
lemmatized_words = [ lemmatize_tupla_word_postag(ow) for ow in tagged_words ]
tagged_words = None
cleaned_words = [ w for w in lemmatized_words if (w not in punctuation) and (w not in stop_words) ]
lemmatized_words = None
return cleaned_words
sentence = "Two electric guitar rocks players, and also a better bass player, are standing off to two sides reading corpora while walking"
print(sentence, "\n\n bag of words:\n", bag_of_words(sentence) )
来源：https://stackoverflow.com/questions/15586721/wordnet-lemmatization-and-pos-tagging-in-python