Amazon AWS EC2 Snapshot Deletion Lambda

import boto3
import collections
import datetime

ec = boto3.client('ec2')

# inspired by https://serverlesscode.com/post/lambda-schedule-ebs-snapshot-backups/
#
# snapshot deletion lambda
#   this function deletes snapshots backed up by the EBSSnapshotterCreator
#   prerequisites:
#     only snapshots for attached volumes are considered
#     the instance that owns the volume must have a Backup=true tag
#     the instance may optionally have a BackupRetentionDays=n tag where n
#        is an integer specifying the number of days to retain backups.
#        if not present, the default is 0
#     this script only considers completed EBS backups for deletion, and it
#     will always retain the most current complete backup.  Only snapshots
#     that have the createdByEBSSnapshotter=true tag will be considered.

def lambda_handler(event, context):
    now = datetime.datetime.now()

    volumeIds = {}

    reservations = ec.describe_instances()['Reservations']
    
    for reservation in reservations:
        for instance in reservation['Instances']:
            backup = False
            backupRetentionDays = 0
            
            for tag in instance['Tags']:
                key = tag['Key']
                value = tag['Value']
                
                if key == 'Backup':
                    backup = tag['Value'] == 'true'
                elif key == 'BackupRetentionDays':
                    backupRetentionDays = int(tag['Value'])

            if not backup: continue

            retentionDate = (now - datetime.timedelta(days=backupRetentionDays)).date()

            for device in instance['BlockDeviceMappings']:
                if device.get('Ebs', None) is None: continue
            
                volumeId = device['Ebs']['VolumeId']

                info = ec.describe_volumes(VolumeIds=[volumeId])['Volumes'][0]
                name = volumeId
                skip = False
                
                for tag in info['Tags']:
                    key = tag['Key']
                    
                    if key == 'Backup':
                        skip = tag['Value'] == 'false'
                    elif key == 'Name':
                        name += ' (' + tag['Value'] + ')'

                if skip:
                    print 'Ignoring snapshots for volume %s due to Backup=false tag' % (name)
                    continue
                else:
                    volumeIds[volumeId] = {
                        'name': name,
                        'retentionDate': retentionDate,
                        'snapshotsToDelete': []
                    }

    accountId = '000000000000' # your 12-digit AWS account id here
    snapshots = ec.describe_snapshots(OwnerIds=[accountId])['Snapshots']

    for snapshot in snapshots:
        if not snapshot['State'] == 'completed': continue

        volumeId = snapshot['VolumeId']
        
        if volumeId not in volumeIds: continue
        
        createdByEBSSnapshotter = False

        tags = snapshot.get('Tags', None)
        
        if tags is None: continue

        for tag in tags:
            if tag['Key'] == 'createdByEBSSnapshotter':
                createdByEBSSnapshotter = tag['Value'] == 'true'
                break
        
        if not createdByEBSSnapshotter: continue
        
        creationDate = snapshot['StartTime'].date()
        retentionDate = volumeIds[volumeId]['retentionDate']

        if creationDate > retentionDate: continue

        snapshotId = snapshot['SnapshotId']

        volumeIds[volumeId]['snapshotsToDelete'].append((snapshotId, creationDate))

    for volumeId in volumeIds.keys():
        name = volumeIds[volumeId]['name']
        snapshotsToDelete = volumeIds[volumeId]['snapshotsToDelete']
        
        if len(snapshotsToDelete) < 2:
            print 'Ignoring snapshots for volume %s because there are fewer than 2 snapshots' % (name)
            continue     # always leave one complete snapshot

        snapshotsToDelete = sorted(snapshotsToDelete, key=lambda creationDate: creationDate[1])
        snapshotsToDelete.pop()

        for snapshot in snapshotsToDelete:
            snapshotId = snapshot[0]
            print 'Deleting snapshot %s for volume %s' % (snapshotId, name)
            ec.delete_snapshot(SnapshotId=snapshotId)