I have similar problem like below
Why did NLTK NaiveBayes classifier misclassify one record?
In my case, I queried positive feed and built positive_vocab and then queried negative feed and built negative_voca. I get the data from feed clean and built the classifier. How do I build the neutral_vocab. Is there a way I can instruct NLTK classifier to return neutral label when the given word is not found in the negative_voca and positive_vocab. How do I do that?
In my current implementation, if I give a word which is not present in the both sets it tells positive by default. Instead it should tell, neutral or notfound