def get_schema_at_timestep(self, schema, timestep):
        """
        Returns the schema of the data at a certain timestep.
        :param schema: The schema of the data, which schema at a certain timestep is a subset of.
        :param timestep: The timestep to get the schema at.
        :return: The schema of the data at a certain timestep.
        """
        return schema.create_schema_view([schema.fields.get(field) for field in schema.fields if
                                          field in self.get_field_names_at_timestep(timestep)])

        Returns the schema of the data at a certain timestep.
        :param schema: The schema of the data, which schema at a certain timestep is a subset of.
        :param timestep: The timestep to get the schema at.
        :return: The schema of the data at a certain timestep.
        
petastorm/ngram.py