Make sure columns are of the same length or else DataFrame construction
        will fail.

def _check_column_lengths(self):
        """
        Make sure columns are of the same length or else DataFrame construction
        will fail.
        """
        column_lengths_dict = {
            name: len(xs)
            for (name, xs)
            in self.columns_dict.items()
        }
        unique_column_lengths = set(column_lengths_dict.values())
        if len(unique_column_lengths) != 1:
            raise ValueError(
                "Mismatch between lengths of columns: %s" % (column_lengths_dict,))