def _train(self, tagged_corpus, cutoff=0, verbose=False):

        token_count = hit_count = 0

        # A context is considered 'useful' if it's not already tagged
        # perfectly by the backoff tagger.
        useful_contexts = set()

        # Count how many times each tag occurs in each context.
        fd = ConditionalFreqDist()
        for sentence in tagged_corpus:
            tokens, tags = zip(*sentence)
            for index, (token, tag) in enumerate(sentence):
                # Record the event.
                token_count += 1
                context = self.context(tokens, index, tags[:index])
                if context is None: continue
                fd[context][tag] += 1
                # If the backoff got it wrong, this context is useful:
                if (self.backoff is None or
                    tag != self.backoff.tag_one(tokens, index, tags[:index])):
                    useful_contexts.add(context)

        # Build the context_to_tag table -- for each context, figure
        # out what the most likely tag is.  Only include contexts that
        # we've seen at least `cutoff` times.
        for context in useful_contexts:
            best_tag = fd[context].max()
            hits = fd[context][best_tag]
            if hits > cutoff:
                self._context_to_tag[context] = best_tag
                hit_count += hits

        # Display some stats, if requested.
        if verbose:
            size = len(self._context_to_tag)
            backoff = 100 - (hit_count * 100.0)/ token_count
            pruning = 100 - (size * 100.0) / len(fd.conditions())
            print("[Trained Unigram tagger:", end=' ')
            print("size=%d, backoff=%.2f%%, pruning=%.2f%%]" % (
                size, backoff, pruning))

######################################################################
#{ Tagger Classes
######################################################################
List all Slack users.
dataset/ETHPy150Open nltk/nltk/nltk/tag/sequential.py/ContextTagger._train--crossover with--dataset/ETHPy150Open saltstack/salt/salt/modules/slack_notify.py/list_users