title: NLTK Lookup Error
body: <p>While running a Python script using NLTK I got this:</p>
  
  <pre><code>Traceback (most recent call last):
    File "cpicklesave.py", line 56, in &lt;module&gt;
      pos = nltk.pos_tag(words)
    File "/usr/lib/python2.7/site-packages/nltk/tag/__init__.py", line 110, in pos_tag
      tagger = PerceptronTagger()
    File "/usr/lib/python2.7/site-packages/nltk/tag/perceptron.py", line 140, in __init__
      AP_MODEL_LOC = str(find('taggers/averaged_perceptron_tagger/'+PICKLE))
    File "/usr/lib/python2.7/site-packages/nltk/data.py", line 641, in find
      raise LookupError(resource_not_found)
  LookupError:
  **********************************************************************
    Resource u'taggers/averaged_perceptron_tagger/averaged_perceptro
    n_tagger.pickle' not found.  Please use the NLTK Downloader to
    obtain the resource:  &gt;&gt;&gt; nltk.download()
    Searched in:
      - '/root/nltk_data'
      - '/usr/share/nltk_data'
      - '/usr/local/share/nltk_data'
      - '/usr/lib/nltk_data'
      - '/usr/local/lib/nltk_data'
  **********************************************************************
  </code></pre>
  
  <p>Can anyone explain the problem?</p>