使用with_statement中的实例的问题

from tencentcloud.cvm.v20170312 import cvm_client, models

# 创建CVM客户端
client = cvm_client.CvmClient(cred, "ap-guangzhou")

# 创建虚拟机实例
def create_instance():
    req = models.RunInstancesRequest()
    # 设置请求参数
    req.ImageId = "img-xxx"
    req.InstanceType = "S2.SMALL1"
    req.InstanceName = "test-instance"
    req.InstanceChargeType = "POSTPAID_BY_HOUR"
    req.SystemDisk.DiskType = "CLOUD_PREMIUM"
    req.SystemDisk.DiskSize = 50
    req.VirtualPrivateCloud.VpcId = "vpc-xxx"
    req.VirtualPrivateCloud.SubnetId = "subnet-xxx"
    req.InternetAccessible.PublicIpAssigned = True
    req.InternetAccessible.InternetMaxBandwidthOut = 1
    req.InstanceCount = 1
    # 发送请求
    resp = client.RunInstances(req)
    # 返回实例ID
    return resp.to_json_string()["Response"]["InstanceIdSet"][0]

# 关闭虚拟机实例
def terminate_instance(instance_id):
    req = models.TerminateInstancesRequest()
    # 设置请求参数
    req.InstanceIds = [instance_id]
    # 发送请求
    client.TerminateInstances(req)

# 使用with语句管理虚拟机实例
with create_instance() as instance_id:
    # 在此处编写使用虚拟机实例的代码
    pass