How to access the event object with python in AWS Lambda?

Question

To follow up on this question: Filter CloudWatch Logs to extract Instance ID

I think it leaves the question incomplete because it does not say how to access the event object with python.

My goal is to:

• read the instance that was triggered by a change in running state
• get a tag value associated with the instance
• start all other instances that have the same tag

The Cloudwatch trigger event is:

{
  "source": [
    "aws.ec2"
  ],
  "detail-type": [
    "EC2 Instance State-change Notification"
  ],
  "detail": {
    "state": [
      "running"
    ]
  }
}

I can see examples like this:

def lambda_handler(event, context):

    # here I want to get the instance tag value
    # and set the tag filter based on the instance that 
    # triggered the event

    filters = [{
            'Name': 'tag:StartGroup',
            'Values': ['startgroup1'] 
        },
        {
            'Name': 'instance-state-name', 
            'Values': ['running']
        }
    ]

    instances = ec2.instances.filter(Filters=filters)

I can see the event object but I don't see how to drill down into the tag of the instance that had it's state changed to running.

Please, what is the object attribute through which I can get a tag from the triggered instance?

I suspect it is something like:

myTag = event.details.instance-id.tags["startgroup1"]

Answer 1

In the Details Section of the Event, you will get the instance Id's. Using the instance id and AWS SDK you can query the tags. The following is the sample event

{
  "version": "0",
  "id": "ee376907-2647-4179-9203-343cfb3017a4",
  "detail-type": "EC2 Instance State-change Notification",
  "source": "aws.ec2",
  "account": "123456789012",
  "time": "2015-11-11T21:30:34Z",
  "region": "us-east-1",
  "resources": [
    "arn:aws:ec2:us-east-1:123456789012:instance/i-abcd1111"
  ],
  "detail": {
    "instance-id": "i-abcd1111",
    "state": "running"
  }
}