title: Read customed corpus in three column format for pos tagging using NLTK
body: <p>I have corpus in txt extention with like this below format:</p>
  <pre><code>Mike NNP B-PERSON
  Noah NNP I-PERSON
  eats VB O
  donuts NN O
  Sarah NNP B-PERSON
  Larsson NNP I-PERSON
  comes VB O
  from IN O
  Stockholm NN B-GPE
  </code></pre>
  <p>I want to read the file to train POS tag (take the words and POS tagger only) like reading a file with a bracket extension (already in a tree format). I tried to change the corpus to something like this in to str format using iteration:</p>
  <pre><code>(NNP Mike) (NNP Noah) (VB eats) (NN donuts) (NNP Sarah) (NNP Larsson) (VB comes) (IN from) (NN Stockholm)
  </code></pre>
  <p>But, when i use tagged_sents() function for it, there is an error:</p>
  <pre><code>'str' object has no attribute 'tagged_sents'
  </code></pre>
  <p>How to read it correctly? Any suggestions? Thank you.</p>