Python Nltk Pos_Tag Not Returning the Correct Part-Of-Speech Tag

Python NLTK pos_tag not returning the correct part-of-speech tag

In short:

NLTK is not perfect. In fact, no model is perfect.

Note:

As of NLTK version 3.1, default pos_tag function is no longer the old MaxEnt English pickle.

It is now the perceptron tagger from @Honnibal's implementation, see nltk.tag.pos_tag

>>> import inspect
>>> print inspect.getsource(pos_tag)
def pos_tag(tokens, tagset=None):
    tagger = PerceptronTagger()
    return _pos_tag(tokens, tagset, tagger)

Still it's better but not perfect:

>>> from nltk import pos_tag
>>> pos_tag("The quick brown fox jumps over the lazy dog".split())
[('The', 'DT'), ('quick', 'JJ'), ('brown', 'NN'), ('fox', 'NN'), ('jumps', 'VBZ'), ('over', 'IN'), ('the', 'DT'), ('lazy', 'JJ'), ('dog', 'NN')]

At some point, if someone wants TL;DR solutions, see https://github.com/alvations/nltk_cli

In long:

Try using other tagger (see https://github.com/nltk/nltk/tree/develop/nltk/tag) , e.g.:

HunPos
Stanford POS
Senna

Using default MaxEnt POS tagger from NLTK, i.e. nltk.pos_tag:

>>> from nltk import word_tokenize, pos_tag
>>> text = "The quick brown fox jumps over the lazy dog"
>>> pos_tag(word_tokenize(text))
[('The', 'DT'), ('quick', 'NN'), ('brown', 'NN'), ('fox', 'NN'), ('jumps', 'NNS'), ('over', 'IN'), ('the', 'DT'), ('lazy', 'NN'), ('dog', 'NN')]

Using Stanford POS tagger:

$ cd ~
$ wget http://nlp.stanford.edu/software/stanford-postagger-2015-04-20.zip
$ unzip stanford-postagger-2015-04-20.zip
$ mv stanford-postagger-2015-04-20 stanford-postagger
$ python
>>> from os.path import expanduser
>>> home = expanduser("~")
>>> from nltk.tag.stanford import POSTagger
>>> _path_to_model = home + '/stanford-postagger/models/english-bidirectional-distsim.tagger'
>>> _path_to_jar = home + '/stanford-postagger/stanford-postagger.jar'
>>> st = POSTagger(path_to_model=_path_to_model, path_to_jar=_path_to_jar)
>>> text = "The quick brown fox jumps over the lazy dog"
>>> st.tag(text.split())
[(u'The', u'DT'), (u'quick', u'JJ'), (u'brown', u'JJ'), (u'fox', u'NN'), (u'jumps', u'VBZ'), (u'over', u'IN'), (u'the', u'DT'), (u'lazy', u'JJ'), (u'dog', u'NN')]

Using HunPOS (NOTE: the default encoding is ISO-8859-1 not UTF8):

$ cd ~
$ wget https://hunpos.googlecode.com/files/hunpos-1.0-linux.tgz
$ tar zxvf hunpos-1.0-linux.tgz
$ wget https://hunpos.googlecode.com/files/en_wsj.model.gz
$ gzip -d en_wsj.model.gz 
$ mv en_wsj.model hunpos-1.0-linux/
$ python
>>> from os.path import expanduser
>>> home = expanduser("~")
>>> from nltk.tag.hunpos import HunposTagger
>>> _path_to_bin = home + '/hunpos-1.0-linux/hunpos-tag'
>>> _path_to_model = home + '/hunpos-1.0-linux/en_wsj.model'
>>> ht = HunposTagger(path_to_model=_path_to_model, path_to_bin=_path_to_bin)
>>> text = "The quick brown fox jumps over the lazy dog"
>>> ht.tag(text.split())
[('The', 'DT'), ('quick', 'JJ'), ('brown', 'JJ'), ('fox', 'NN'), ('jumps', 'NNS'), ('over', 'IN'), ('the', 'DT'), ('lazy', 'JJ'), ('dog', 'NN')]

Using Senna (Make sure you've the latest version of NLTK, there were some changes made to the API):

$ cd ~
$ wget http://ronan.collobert.com/senna/senna-v3.0.tgz
$ tar zxvf senna-v3.0.tgz
$ python
>>> from os.path import expanduser
>>> home = expanduser("~")
>>> from nltk.tag.senna import SennaTagger
>>> st = SennaTagger(home+'/senna')
>>> text = "The quick brown fox jumps over the lazy dog"
>>> st.tag(text.split())
[('The', u'DT'), ('quick', u'JJ'), ('brown', u'JJ'), ('fox', u'NN'), ('jumps', u'VBZ'), ('over', u'IN'), ('the', u'DT'), ('lazy', u'JJ'), ('dog', u'NN')]

Or try building a better POS tagger:

Ngram Tagger: http://streamhacker.com/2008/11/03/part-of-speech-tagging-with-nltk-part-1/
Affix/Regex Tagger: http://streamhacker.com/2008/11/10/part-of-speech-tagging-with-nltk-part-2/
Build Your Own Brill (Read the code it's a pretty fun tagger, http://www.nltk.org/_modules/nltk/tag/brill.html), see http://streamhacker.com/2008/12/03/part-of-speech-tagging-with-nltk-part-3/
Perceptron Tagger: https://honnibal.wordpress.com/2013/09/11/a-good-part-of-speechpos-tagger-in-about-200-lines-of-python/
LDA Tagger: http://scm.io/blog/hack/2015/02/lda-intentions/

Complains about pos_tag accuracy on stackoverflow include:

POS tagging - NLTK thinks noun is adjective
python NLTK POS tagger not behaving as expected
How to obtain better results using NLTK pos tag
pos_tag in NLTK does not tag sentences correctly

Issues about NLTK HunPos include:

How do I tag textfiles with hunpos in nltk?
Does anyone know how to configure the hunpos wrapper class on nltk?

Issues with NLTK and Stanford POS tagger include:

trouble importing stanford pos tagger into nltk
Java Command Fails in NLTK Stanford POS Tagger
Error using Stanford POS Tagger in NLTK Python
How to improve speed with Stanford NLP Tagger and NLTK
Nltk stanford pos tagger error : Java command failed
Instantiating and using StanfordTagger within NLTK
Running Stanford POS tagger in NLTK leads to "not a valid Win32 application" on Windows

pos_tag in NLTK does not tag sentences correctly

Short answer: you can't. Slightly longer answer: you can override specific words using a manually created UnigramTagger. See my answer for custom tagging with nltk for details on this method.