title: Lemmatize words inside lists in pandas dataframe
body: <p>After applying tokenizing, I have a pandas data frame as shown below. I want to apply the nltk lemmatizer in this data frame. What I tried is give here. I am getting error saying '<strong>if form in exceptions:TypeError: unhashable type: 'list'</strong>'. How can I properly implement the lemmatizer here?</p>
  
  <p>Also please note that the 5th data frame cell has an empty list. How can I remove such lists in this data frame? </p>
  
  <pre><code> [[ive, searching, right, words, thank, breather], [i, promise, wont, take, help, granted, fulfil, promise], [you, wonderful, blessing, times]]                     
  
   [[free, entry, 2, wkly, comp, win, fa, cup, final, tkts, 21st, may, 2005], [text, fa, 87121, receive, entry, questionstd, txt, ratetcs, apply, 08452810075over18s]]
  
   [[nah, dont, think, goes, usf, lives, around, though]]                                                                                                             
  
   [[even, brother, like, speak, me], [they, treat, like, aids, patent]]                                                                                              
  
   [[i, date, sunday, will], []] 
  </code></pre>
  
  <blockquote>
    <p>The lemmatizer function I tried</p>
  </blockquote>
  
  <pre><code>def lemmatize(fullCorpus):
      lemmatizer = nltk.stem.WordNetLemmatizer()
      lemmatized = fullCorpus['tokenized'].apply(lambda row: list(map([lemmatizer.lemmatize(y) for y in row])))
      return lemmatized
  </code></pre>