Convert descriptor to BigQuery

def convert_descriptor(self, descriptor):
        """Convert descriptor to BigQuery
        """

        # Fields
        fields = []
        fallbacks = []
        schema = tableschema.Schema(descriptor)
        for index, field in enumerate(schema.fields):
            converted_type = self.convert_type(field.type)
            if not converted_type:
                converted_type = 'STRING'
                fallbacks.append(index)
            mode = 'NULLABLE'
            if field.required:
                mode = 'REQUIRED'
            fields.append({
                'name': _slugify_field_name(field.name),
                'type': converted_type,
                'mode': mode,
            })

        # Descriptor
        converted_descriptor = {
            'fields': fields,
        }

        return (converted_descriptor, fallbacks)