title: Python: 'Nontype' object has no attribute keys
body: <pre><code>   def index_dir(self, base_path): 
  
          num_files_indexed = 0
          allfiles = os.listdir(base_path)
          #print allfiles
          num_files_indexed = len(allfiles)
          #print num_files_indexed
          docnumber = 0
          self._inverted_index = {} #dictionary
          for file in allfiles: 
                  self.documents = [base_path+file] #list of all text files
                  f = open(base_path+file, 'r')
                  lines = f.read()
    # Tokenize the file into words
                  tokens = self.tokenize(lines)
                  docnumber = docnumber + 1
                  print 'docnumber', docnumber
                  for term in tokens:  
  # check if the key already exists in the dictionary, if yes, 
  # just add a new value for the key
                      #if self._inverted_index.has_key(term)
                      if term in sorted(self._inverted_index.keys()):
                          docnumlist = self._inverted_index.get(term)
                          docnumlist = docnumlist.append(docnumber)
                      else:
  # if the key doesn't exist in dictionary, add the key (term) 
  # and associate the docnumber value with it. 
                          self._inverted_index = self._inverted_index.update({term: docnumber})
     #self._inverted_index[term] = docnumber 
                  f.close()
          print 'dictionary', self._inverted_index 
          print 'keys', self._inverted_index.keys()
          return num_files_indexed
  </code></pre>
  
  <p>I'm working on an information retrieval project where we are supposed to crawl through multiple text file, tokenize the files and store the words in an inverted list (dictionary) data structure. </p>
  
  <p>ex:
  doc1.txt: "the dog ran"
  doc2.txt: "the cat slept"<br>
  _inverted_index = {
               'the': [0,1],
               'dog': [0],
               'ran': [0],
               'cat': [1],
               'slept': [1]
               }
  where 0,1 are docIDs</p>
  
  <p>I'm getting the following error:
  'Nontype' object has no attribute keys. line#95</p>
  
  <p>All help is highly appreciated.</p>