AWS AMI creation Python code

# Automated AMI Backups
#
# @author Robert Kozora <bobby@kozora.me> [I made some changes.]
#
# This script will search for all instances from all region and 
# as soon as we have the instances list, we loop through each instance
# and create an AMI of it. Also, it will look for a "Retention" tag key which
# will be used as a retention policy number in days. If there is no tag with
# that name, it will use a 7 days default value for each AMI.
#
# After creating the AMI it creates a "DeleteOn" tag on the AMI indicating when
# it will be deleted using the Retention value and another Lambda function

import boto3
import collections
import datetime
import sys
import pprint

ec = boto3.client('ec2')
#image = ec.Image('id')

def lambda_handler(event, context):

    reservations = ec.describe_instances(

    ).get(
        'Reservations', []
    )

    instances = sum(
        [
            [i for i in r['Instances']]
            for r in reservations
        ], [])

    print "Found %d instances that need backing up" % len(instances)

    to_tag = collections.defaultdict(list)

    for instance in instances:
        try:
            retention_days = [
                int(t.get('Value')) for t in instance['Tags']
                if t['Key'] == 'Retention'][0]
        except IndexError:
            retention_days = 3

        #for dev in instance['BlockDeviceMappings']:
        #    if dev.get('Ebs', None) is None:
        #        continue
        #    vol_id = dev['Ebs']['VolumeId']
        #    print "Found EBS volume %s on instance %s" % (
        #        vol_id, instance['InstanceId'])

            #snap = ec.create_snapshot(
            #    VolumeId=vol_id,
            #)

            #create_image(instance_id, name, description=None, no_reboot=False, block_device_mapping=None, dry_run=False)
            # DryRun, InstanceId, Name, Description, NoReboot, BlockDeviceMappings
            create_time = datetime.datetime.now()
            create_fmt = create_time.strftime('%Y-%m-%d--%H-%M-%S')

            AMIid = ec.create_image(InstanceId=instance['InstanceId'], Name="Lambda - " + instance['InstanceId'] + " from " + create_fmt, Description="Lambda created AMI of instance " + instance['InstanceId'] + " from " + create_fmt, NoReboot=True, DryRun=False)


            pprint.pprint(instance)
            #sys.exit()
            #break

            #to_tag[retention_days].append(AMIid)

            to_tag[retention_days].append(AMIid['ImageId'])

            print "Retaining AMI %s of instance %s for %d days" % (
                AMIid['ImageId'],
                instance['InstanceId'],
                retention_days,
            )

    print to_tag.keys()

    for retention_days in to_tag.keys():
        delete_date = datetime.date.today() + datetime.timedelta(days=retention_days)
        delete_fmt = delete_date.strftime('%m-%d-%Y')
        print "Will delete %d AMIs on %s" % (len(to_tag[retention_days]), delete_fmt)

        #break

        ec.create_tags(
            Resources=to_tag[retention_days],
            Tags=[
                {'Key': 'DeleteOn', 'Value': delete_fmt},
            ]
)