Return a dictionary of statistics collected during creation of the
        model. These statistics are also available with the ``get`` method and
        are described in more detail in that method's documentation.

        Returns
        -------
        out : dict
            Dictionary of statistics compiled during creation of the
            TopicModel.

        See Also
        --------
        summary

        Examples
        --------
        >>> docs = turicreate.SArray('https://static.turi.com/datasets/nips-text')
        >>> m = turicreate.topic_model.create(docs)
        >>> m._training_stats()
        {'training_iterations': 20,
         'training_time': 20.5034}

def _training_stats(self):
        """
        Return a dictionary of statistics collected during creation of the
        model. These statistics are also available with the ``get`` method and
        are described in more detail in that method's documentation.

        Returns
        -------
        out : dict
            Dictionary of statistics compiled during creation of the
            TopicModel.

        See Also
        --------
        summary

        Examples
        --------
        >>> docs = turicreate.SArray('https://static.turi.com/datasets/nips-text')
        >>> m = turicreate.topic_model.create(docs)
        >>> m._training_stats()
        {'training_iterations': 20,
         'training_time': 20.5034}
        """

        fields = self._list_fields()
        stat_fields = ['training_time',
                       'training_iterations']
        if 'validation_perplexity' in fields:
            stat_fields.append('validation_perplexity')

        ret = {k : self._get(k) for k in stat_fields}
        return ret