如何模拟特定的boto3异常？

Question

我想在cloudformation客户端引发AlreadyExistsException时测试我的代码行为。但是我不知道如何用python模拟来提高它。

class StackHelper:

    def __init__(self, stack_name: str, cfn_client, capabilities: List[str]):
        self.stack_name = stack_name
        self.cfn_client = cfn_client
        self.capabilities = capabilities

    def deploy(self, template_url: str, params: object = object()):
        try:
            self.create_stack(template_url, params)
        except self.cfn_client.exceptions.AlreadyExistsException as e:
            try:
                print('Stack already exists => update_stack')
                self.update_stack(template_url, params)
            except ClientError as e:
                if e.response['Error']['Message'] == 'No updates are to be performed.':
                    print('Nothing to update.')
                else:
                    raise

测试类似于：

    mock_cfn_client = Mock()
    mock_cfn_client.return_value.create_stack.side_effect = Mock(side_effect=???)

为此，我已经嘲笑了cfn_client，但是现在却在如何引发我期望在except self.cfn_client.exceptions.AlreadyExistsException的代码中遇到的boto3异常方面苦苦挣扎。