Python NLTK POS tagger workaround example.

nltk_pos_workaround.py

import nltk
import re
from nltk.tokenize import word_tokenize, sent_tokenize

text = "I'm not going to the party."

words = word_tokenize(text)
pos_tags = nltk.pos_tag(words)

print (pos_tags)
for index, tag in enumerate(pos_tags):
    match = re.search('\w.*', tag[1])
    if match:
        if tag[0] == "n't" or tag[0] == "not":
            pos_tags[index] = (pos_tags[index][0], "NEG")

print (pos_tags)

nltk_pos_workaround_output

[('I', 'PRP'), ("'m", 'VBP'), ('not', 'RB'), ('going', 'VBG'), ('to', 'TO'), ('the', 'DT'), ('party', 'NN'), ('.', '.')]
[('I', 'PRP'), ("'m", 'VBP'), ('not', 'NEG'), ('going', 'VBG'), ('to', 'TO'), ('the', 'DT'), ('party', 'NN'), ('.', '.')]