Source code for nltk.classify.naivebayes

# Natural Language Toolkit: Naive Bayes Classifiers
#
# Copyright (C) 2001-2014 NLTK Project
# Author: Edward Loper <edloper@gmail.com>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT

"""
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) = ------------------------------
|                              P(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) = --------------------------------------------
|                                         P(features)

Rather than computing P(featues) explicitly, the algorithm just
calculates the denominator 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) )
"""
from __future__ import print_function, unicode_literals

from collections import defaultdict

from nltk.probability import FreqDist, DictionaryProbDist, ELEProbDist, sum_logs
from nltk.classify.api import ClassifierI

##//////////////////////////////////////////////////////
##  Naive Bayes Classifier
##//////////////////////////////////////////////////////

[docs]class NaiveBayesClassifier(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. """ def __init__(self, label_probdist, feature_probdist): """ :param 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)``. :param 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``. """ self._label_probdist = label_probdist self._feature_probdist = feature_probdist self._labels = list(label_probdist.samples())
[docs] def labels(self): return self._labels
[docs] def classify(self, featureset): return self.prob_classify(featureset).max()
[docs] def prob_classify(self, featureset): # Discard any feature names that we've never seen before. # Otherwise, we'll just assign a probability of 0 to # everything. featureset = featureset.copy() for fname in list(featureset.keys()): for label in self._labels: if (label, fname) in self._feature_probdist: break else: #print 'Ignoring unseen feature %s' % fname del featureset[fname] # Find the log probabilty of each label, given the features. # Start with the log probability of the label itself. logprob = {} for label in self._labels: logprob[label] = self._label_probdist.logprob(label) # Then add in the log probability of features given labels. for label in self._labels: for (fname, fval) in featureset.items(): if (label, fname) in self._feature_probdist: feature_probs = self._feature_probdist[label,fname] logprob[label] += feature_probs.logprob(fval) else: # nb: This case will never come up if the # classifier was created by # NaiveBayesClassifier.train(). logprob[label] += sum_logs([]) # = -INF. return DictionaryProbDist(logprob, normalize=True, log=True)
[docs] def show_most_informative_features(self, n=10): # Determine the most relevant features, and display them. cpdist = self._feature_probdist print('Most Informative Features') for (fname, fval) in self.most_informative_features(n): def labelprob(l): return cpdist[l,fname].prob(fval) labels = sorted([l for l in self._labels if fval in cpdist[l,fname].samples()], key=labelprob) if len(labels) == 1: continue l0 = labels[0] l1 = labels[-1] if cpdist[l0,fname].prob(fval) == 0: ratio = 'INF' else: ratio = '%8.1f' % (cpdist[l1,fname].prob(fval) / cpdist[l0,fname].prob(fval)) print(('%24s = %-14r %6s : %-6s = %s : 1.0' % (fname, fval, ("%s" % l1)[:6], ("%s" % l0)[:6], ratio)))
[docs] def most_informative_features(self, n=100): """ 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) ] """ # The set of (fname, fval) pairs used by this classifier. features = set() # The max & min probability associated w/ each (fname, fval) # pair. Maps (fname,fval) -> float. maxprob = defaultdict(lambda: 0.0) minprob = defaultdict(lambda: 1.0) for (label, fname), probdist in self._feature_probdist.items(): for fval in probdist.samples(): feature = (fname, fval) features.add( feature ) p = probdist.prob(fval) maxprob[feature] = max(p, maxprob[feature]) minprob[feature] = min(p, minprob[feature]) if minprob[feature] == 0: features.discard(feature) # Convert features to a list, & sort it by how informative # features are. features = sorted(features, key=lambda feature_: minprob[feature_]/maxprob[feature_]) return features[:n]
@staticmethod
[docs] def train(labeled_featuresets, estimator=ELEProbDist): """ :param labeled_featuresets: A list of classified featuresets, i.e., a list of tuples ``(featureset, label)``. """ label_freqdist = FreqDist() feature_freqdist = defaultdict(FreqDist) feature_values = defaultdict(set) fnames = set() # Count up how many times each feature value occurred, given # the label and featurename. for featureset, label in labeled_featuresets: label_freqdist[label] += 1 for fname, fval in featureset.items(): # Increment freq(fval|label, fname) feature_freqdist[label, fname][fval] += 1 # Record that fname can take the value fval. feature_values[fname].add(fval) # Keep a list of all feature names. fnames.add(fname) # If a feature didn't have a value given for an instance, then # we assume that it gets the implicit value 'None.' This loop # counts up the number of 'missing' feature values for each # (label,fname) pair, and increments the count of the fval # 'None' by that amount. for label in label_freqdist: num_samples = label_freqdist[label] for fname in fnames: count = feature_freqdist[label, fname].N() # Only add a None key when necessary, i.e. if there are # any samples with feature 'fname' missing. if num_samples - count > 0: feature_freqdist[label, fname][None] += num_samples - count feature_values[fname].add(None) # Create the P(label) distribution label_probdist = estimator(label_freqdist) # Create the P(fval|label, fname) distribution feature_probdist = {} for ((label, fname), freqdist) in feature_freqdist.items(): probdist = estimator(freqdist, bins=len(feature_values[fname])) feature_probdist[label,fname] = probdist return NaiveBayesClassifier(label_probdist, feature_probdist) ##////////////////////////////////////////////////////// ## Demo ##//////////////////////////////////////////////////////
[docs]def demo(): from nltk.classify.util import names_demo classifier = names_demo(NaiveBayesClassifier.train) classifier.show_most_informative_features()
if __name__ == '__main__': demo()