':param      id: The ID of the subnet association in the VPC.
:type       id: ``str``
:param      route_table_id: The ID of a route table in the VPC.
:type       route_table_id: ``str``
:param      subnet_id: The ID of a subnet in the VPC.
:type       subnet_id: ``str``
:param      main: If true, means this is a main VPC route table.
:type       main: ``bool``'

def __init__(self, id, route_table_id, subnet_id, main=False):

    self.id = id
    self.route_table_id = route_table_id
    self.subnet_id = subnet_id
    self.main = main
