classify.py

    def classify(self, instance):
        '''
        given a single observation, will return the output of the tree
        '''
        attr = self.decision_attribute

        if self.label is not None:
            return self.label
        elif self.splitting_value:
            if instance[attr] < self.splitting_value:
                return self.children[0].classify(instance)
            else:
                return self.children[1].classify(instance)
        else:
            key = instance[attr]
            try:
                child = self.children[str(key) if key is None else key]
            except KeyError: # Oh no! something in the validation set we've
                # never seen before; probably (hopefully) a None
                key = random.choice(self.children.keys())
                child = self.children[key]
            return child.classify(instance)