code: def get_all_dbinstances(self, instance_id=None, max_records=None,
                              marker=None):
          
          params = {}
          if instance_id:
              params['DBInstanceIdentifier'] = instance_id
          if max_records:
              params['MaxRecords'] = max_records
          if marker:
              params['Marker'] = marker
          return self.get_list('DescribeDBInstances', params,
                               [('DBInstance', DBInstance)])
docs: Retrieve all the DBInstances in your account.
  
          :type instance_id: str
          :param instance_id: DB Instance identifier.  If supplied, only
                              information this instance will be returned.
                              Otherwise, info about all DB Instances will
                              be returned.
  
          :type max_records: int
          :param max_records: The maximum number of records to be returned.
                              If more results are available, a MoreToken will
                              be returned in the response that can be used to
                              retrieve additional records.  Default is 100.
  
          :type marker: str
          :param marker: The marker provided by a previous request.
  
          :rtype: list
          :return: A list of :class:`boto.rds.dbinstance.DBInstance`