code: def get_filter_value(self, filter_name):
          
          if filter_name in ('cidr', 'cidrBlock', 'cidr-block'):
              return self.cidr_block
          elif filter_name in ('vpc-id', 'vpcId'):
              return self.vpc_id
          elif filter_name == 'subnet-id':
              return self.id
          elif filter_name in ('availabilityZone', 'availability-zone'):
              return self.availability_zone
          elif filter_name in ('defaultForAz', 'default-for-az'):
              return self.default_for_az
          else:
              return super(Subnet, self).get_filter_value(
                  filter_name, 'DescribeSubnets')
docs: API Version 2014-10-01 defines the following filters for DescribeSubnets:
  
          * availabilityZone
          * available-ip-address-count
          * cidrBlock
          * defaultForAz
          * state
          * subnet-id
          * tag:key=value
          * tag-key
          * tag-value
          * vpc-id
  
          Taken from: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeSubnets.html