List all ECS task definitions not in use (i.e. not belonging to a task running in any cluster)

unused_task_definitions.py

import re
import boto3

client = boto3.client('ecs', region_name='us-east-1')
used_tds = {re.match(r'arn:aws:ecs:us-east-1:667583086810:task-definition/([\w-]+):\d+$',
                     s['taskDefinition']
                     ).group(1)
            for c in client.list_clusters()['clusterArns']
            for s in
            client.describe_services(cluster=c, services=client.list_services(cluster=c)['serviceArns'])['services']
            }

all_tds = set(client.list_task_definition_families()['families'])
for td in sorted(all_tds - used_tds):
    print td