Note: Could import scipy.stats and use scipy.stats.entropy(doc_freq, summary_freq)
        but this gives equivalent value without the import

def _kl_divergence(self, summary_freq, doc_freq):
        """
        Note: Could import scipy.stats and use scipy.stats.entropy(doc_freq, summary_freq)
        but this gives equivalent value without the import
        """
        sum_val = 0
        for w in summary_freq:
            frequency = doc_freq.get(w)
            if frequency:  # missing or zero = no frequency
                sum_val += frequency * math.log(frequency / summary_freq[w])

        return sum_val