Python: 'Nontype' object has no attribute keys

Question

   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

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.

ex: doc1.txt: "the dog ran" doc2.txt: "the cat slept"
_inverted_index = { 'the': [0,1], 'dog': [0], 'ran': [0], 'cat': [1], 'slept': [1] } where 0,1 are docIDs

I'm getting the following error: 'Nontype' object has no attribute keys. line#95

All help is highly appreciated.

Answer 1

When self._inverted_index is a dictionary, self._inverted_index.update will update it in-place and return None (like most mutators do). So, the disastrous bug in your code is the line:

 self._inverted_index = self._inverted_index.update({term: docnumber})

which sets self._inverted_index to None. Just change it to

 self._inverted_index.update({term: docnumber})

simply accepting the in-place update (mutation) and without the erroneous assignment!