nltk.classify.naivebayes module

A classifier based on the Naive Bayes algorithm. In order to find the probability for a label, this algorithm first uses the Bayes rule to express P(label|features) in terms of P(label) and P(features|label):

P(label) * P(features|label)
P(label|features) = ——————————

The algorithm then makes the ‘naive’ assumption that all features are independent, given the label:

P(label) * P(f1|label) * … * P(fn|label)
P(label|features) = ——————————————–

Rather than computing P(features) explicitly, the algorithm just calculates the numerator for each label, and normalizes them so they sum to one:

P(label) * P(f1|label) * … * P(fn|label)
P(label|features) = ——————————————–
SUM[l]( P(l) * P(f1|l) * … * P(fn|l) )
class nltk.classify.naivebayes.NaiveBayesClassifier[source]

Bases: nltk.classify.api.ClassifierI

A Naive Bayes classifier. Naive Bayes classifiers are paramaterized by two probability distributions:

  • P(label) gives the probability that an input will receive each label, given no information about the input’s features.

  • P(fname=fval|label) gives the probability that a given feature (fname) will receive a given value (fval), given that the label (label).

If the classifier encounters an input with a feature that has never been seen with any label, then rather than assigning a probability of 0 to all labels, it will ignore that feature.

The feature value ‘None’ is reserved for unseen feature values; you generally should not use ‘None’ as a feature value for one of your own features.

__init__(label_probdist, feature_probdist)[source]
  • label_probdist – P(label), the probability distribution over labels. It is expressed as a ProbDistI whose samples are labels. I.e., P(label) = label_probdist.prob(label).

  • feature_probdist – P(fname=fval|label), the probability distribution for feature values, given labels. It is expressed as a dictionary whose keys are (label, fname) pairs and whose values are ProbDistI objects over feature values. I.e., P(fname=fval|label) = feature_probdist[label,fname].prob(fval). If a given (label,fname) is not a key in feature_probdist, then it is assumed that the corresponding P(fname=fval|label) is 0 for all values of fval.


the list of category labels used by this classifier.

Return type

list of (immutable)


the most appropriate label for the given featureset.

Return type



a probability distribution over labels for the given featureset.

Return type



Return a list of the ‘most informative’ features used by this classifier. For the purpose of this function, the informativeness of a feature (fname,fval) is equal to the highest value of P(fname=fval|label), for any label, divided by the lowest value of P(fname=fval|label), for any label:

max[ P(fname=fval|label1) / P(fname=fval|label2) ]
classmethod train(labeled_featuresets, estimator=<class 'nltk.probability.ELEProbDist'>)[source]

labeled_featuresets – A list of classified featuresets, i.e., a list of tuples (featureset, label).