原始的boto关于ElasticIP的api使用起来比较不友好，例如实例与EIP关联的时候，需要给出VPC里的EIP的 allocation_id，而allocation_id要从eip的属性里找出来，disassociate eip的时候，需要提供association_id，这个id比较难找。而常规的思路是，不管是关联和解关联，只需要提供实例ID和EIP就行，因此我 封装了几个函数来使EIP的使用变的稍微友好点。
1，将eip的allocation函数封装，返回IP和allocationIP的字典，供后面关联函数使用

import boto.ec2
region = 'ap-southeast-1'

def eip_allocation(domain='vpc'):
    conn = boto.ec2.connect_to_region(region)
    allocation = conn.allocate_address(domain='vpc', dry_run=False)
    return {'public_ip':allocation.public_ip,
            'allocation_id':allocation.allocation_id}

返回字典例如
{'public_ip':'54.169.xx.xx', 'allocation_id':'eipalloc-b2e3xxxx'}

2，原始的associate函数需要给出实例ID和EIP的allocation_id，那假如不是分配IP后立即使用，而是把已经存在的EIP 与实例关联，这样就得通过EIP来解析出allocation_id，于是重新包装下，使其只需要实例ID和EIP即可，而eip_allocation 函数正好二者都返回，随便选用。

def eip_association(instance_id,public_ip):
    '''Notice: need InstanceID and PublicIP to make association'''
    conn = boto.ec2.connect_to_region(region)
    ##make sure input ip is valid EIP
    try:
        address = conn.get_all_addresses(addresses=[public_ip])
    except boto.exception.EC2ResponseError:
        print "Error: IP not found or not EIP"
    else:
        allocation_id = address[0].allocation_id
    ## to call boto associate_address func
    conn.associate_address(
        instance_id=instance_id,
        public_ip=None,
        allocation_id=allocation_id,
        network_interface_id=None,
        private_ip_address=None,
        allow_reassociation=False,
        dry_run=False)

3，disassociate原始的函数也是需要association_id的，而正常思路是只需要提供EIP就可以将其与实例解除关联，于是重新包装了函数，只需要给出EIP就能解除关联

def eip_disassociation(public_ip):
    conn = boto.ec2.connect_to_region(region)
    '''Notice: use public_ip to get network_interface_id
    and use network_interface_id to get association_id'''
    ##make sure input ip is valid EIP
    try:
        address = conn.get_all_addresses(addresses=[public_ip])
    except boto.exception.EC2ResponseError:
        print "Error: IP not found or not EIP"
    else:
        association_id = address[0].association_id
        #network_interface_id = address[0].network_interface_id    
    conn.disassociate_address(
        public_ip=None,
        association_id=association_id,
        dry_run=False)

4，释放EIP，重新包装了下，使其不需要allocation_id，只需要给出EIP就能释放，并且会判断EIP是否被使用，只有没被使用的EIP才能被释放，当然，每个函数还有个判断输入的EIP是否为真实的EIP。

def eip_release(public_ip):
    conn = boto.ec2.connect_to_region(region)
    ##make sure input ip is valid EIP
    try:
        address = conn.get_all_addresses(addresses=[public_ip])
    except boto.exception.EC2ResponseError:
        print "Error: IP not found or not EIP"
    else:
        association_id = address[0].association_id
        allocation_id = address[0].allocation_id
    ## only release EIP that not associated to any instance
    if association_id == None:
        conn.release_address(
            public_ip=None,
            allocation_id=allocation_id)
    else:
        print "IP %s is in use, cannot be released" % public_ip

5，重新构造了个函数，用户给实例快速更换EIP，适用于爬虫服务器，因为他的IP经常被封掉。另外提一句，如果实例创建之初自动分配过 Public IP的话，关联EIP之后，PublicIP也会变成与ElasticIP一样（被覆盖），等解除EIP关联之后，PublicIP才会显露，但此时会重 新分配，因此PublicIP会变。

def eip_change(instance_id):
    conn = boto.ec2.connect_to_region(region)
    reservations = conn.get_all_reservations([instance_id])
    instances = reservations[0].instances
    inst = instances[0]
    old_public_ip = inst.ip_address
    ##make sure the public ip is valid EIP
    try:
        address = conn.get_all_addresses(addresses=[old_public_ip])
    except boto.exception.EC2ResponseError:
        print "Error: Old public IP not found or not EIP"
    else:
        eip_disassociation(old_public_ip)
        eip_release(old_public_ip)
    ## add new IP 
    allocation = eip_allocation()
    public_ip = allocation['public_ip']
    eip_association(instance_id,public_ip)

6，下面是主函数里的调用示例

def main():
    #usage demos
    #eip_disassociation('54.xx.xx.xx')
    #eip_association('i-xxxxxxxx','54.xx.xx.xx')
    #eip_release('54.xx.xx.xx')
    #eip_change('i-xxxxxxxx')
     
if __name__ == '__main__':
    main()