<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _set_description(self, schema):
 """Set description from schema. :type schema: Sequence[google.cloud.bigquery.schema.SchemaField] :param schema: A description of fields in the schema. """

        if schema is None:
            self.description = None
            return

        self.description = tuple(
            [
                Column(
                    name=field.name,
                    type_code=field.field_type,
                    display_size=None,
                    internal_size=None,
                    precision=None,
                    scale=None,
                    null_ok=field.is_nullable,
                )
                for field in schema
            ]
        )