AWS lambda to start/stop tagged EC2 instances

state_manager.py

import sys, argparse
import boto3

# { "action" : "stop", "tag" : "Managed",  "value" :  "True", "dryrun" : false }
from argparse import RawDescriptionHelpFormatter

def __manage(event):
    __log("PARAMS\taction: %s\tTag: %s=%s\tDryRun=%s" %
          (event['action'], event['tag'], event['value'], str(event['dryrun'])))
    ec2 = boto3.resource('ec2')
    tagfilter = [{
        'Name': ("tag:%s" % event['tag']),
        'Values': [event['value']]
    }]
    state = "stopped" if (event['action'] == "start") else "running";

    for i in ec2.instances.filter(Filters=tagfilter):
        if (i.state['Name'] == state):
            if (event['dryrun']):
                __log("DRYRUN\taction: %s\t%s" % (event['action'], i))
            else:
                __log("APPLY\taction: %s\t%s" % (event['action'], i))
                if (event['action'] == "start"):
                    i.start()
                else:
                    i.stop()
        else:
            __log("NOOP \taction: %s\t%s\t instance in state: %s" % (event['action'], i, i.state))


def __log(str):
    print(str)

def lambda_handler(event, context):
    """expects parameters
        :param event:   hash with 'action', 'tag', 'value', and 'dryrun' keys
        :param context:
    :return:
    """
    __manage(event)

def main(args):
    description = """
    look for all instances with the given tag value and apply the given state change 
    unless the instance is already in the given state 
"""
    parser = argparse.ArgumentParser(description=description,
                                     formatter_class=RawDescriptionHelpFormatter)
    parser.add_argument('-t', '--tag', required=True, help='name of tag to look for')
    parser.add_argument('-V', '--value', required=True, help='tag value to look for  (string)')
    parser.add_argument('-y', '--dryrun', action='store_true',
                        help='dry run only - do not apply given action to matching instances')
    parser.add_argument('action', nargs=1, help='start or stop')

    args = parser.parse_args()
    event = { 'tag': args.tag, 'value' : args.value, 'dryrun' : args.dryrun, 'action' : args.action[0]}
    __manage(event)


if __name__ == "__main__":
    sys.exit(main(sys.argv))