title: How to extract nouns from dataframe
body: <p>I want to extract nouns from dataframe. Only nouns.
  I do as below</p>
  
  <pre><code>import pandas as pd
  import nltk
  from nltk.tag import pos_tag
  from nltk import word_tokenize
  df = pd.DataFrame({'noun': ['good day', 'good night']})
  </code></pre>
  
  <p>I want to get</p>
  
  <pre><code>    noun
  0   day
  1   night
  </code></pre>
  
  <p>My code    </p>
  
  <pre><code>df['noun'] = df.apply(lambda row: nltk.word_tokenize(row['noun']), axis=1) 
  noun=[]
  for  index, row in df.iterrows():
      noun.append([word for word,pos in pos_tag(row) if pos == 'NN'])
  df['noun'] = noun 
  
  
  
   ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-194-688cfbb21ec5&gt; in &lt;module&gt;()
        1 noun=[]
        2 for  index, row in df.iterrows():
  ----&gt; 3     noun.append([word for word,pos in pos_tag(row) if pos == 'NN'])
        4 df['noun'] = noun
  
  C:\Users\Edward\Anaconda3\lib\site-packages\nltk\tag\__init__.py in pos_tag(tokens, tagset)
      109     """
      110     tagger = PerceptronTagger()
  --&gt; 111     return _pos_tag(tokens, tagset, tagger)
      112 
      113 
  
  C:\Users\Edward\Anaconda3\lib\site-packages\nltk\tag\__init__.py in _pos_tag(tokens, tagset, tagger)
       80 
       81 def _pos_tag(tokens, tagset, tagger):
  ---&gt; 82     tagged_tokens = tagger.tag(tokens)
       83     if tagset:
       84         tagged_tokens = [(token, map_tag('en-ptb', tagset, tag)) for (token, tag) in tagged_tokens]
  
  C:\Users\Edward\Anaconda3\lib\site-packages\nltk\tag\perceptron.py in tag(self, tokens)
      150         output = []
      151 
  --&gt; 152         context = self.START + [self.normalize(w) for w in tokens] + self.END
      153         for i, word in enumerate(tokens):
      154             tag = self.tagdict.get(word)
  
  C:\Users\Edward\Anaconda3\lib\site-packages\nltk\tag\perceptron.py in &lt;listcomp&gt;(.0)
      150         output = []
      151 
  --&gt; 152         context = self.START + [self.normalize(w) for w in tokens] + self.END
      153         for i, word in enumerate(tokens):
      154             tag = self.tagdict.get(word)
  
  C:\Users\Edward\Anaconda3\lib\site-packages\nltk\tag\perceptron.py in normalize(self, word)
      222         if '-' in word and word[0] != '-':
      223             return '!HYPHEN'
  --&gt; 224         elif word.isdigit() and len(word) == 4:
      225             return '!YEAR'
      226         elif word[0].isdigit():
  
  AttributeError: 'list' object has no attribute 'isdigit'
  </code></pre>
  
  <p>Please, help, how to improve it?
  * Sorry, i have ro write some text so that i can insert all traceback
  I guess thr problem is that i cann't convert list to needed format?</p>