unused_subnets.py

#/usr/bin/env python3

import boto3
from termcolor import colored
subnetsIDs={}
ec2 = boto3.resource('ec2')
client = boto3.client('ec2')
response=client.describe_subnets()

for i in response['Subnets']:
    subnetsIDs[i['SubnetId']]=0

response=client.describe_nat_gateways()

for i in response['NatGateways']:
	subnetsIDs[i['SubnetId']]=1
	
client = boto3.client('rds')
response = client.describe_db_instances()

for i in response['DBInstances']:
    for x in i['DBSubnetGroup']['Subnets']:
        subnetsIDs[x['SubnetIdentifier']]=1

client = boto3.client('autoscaling')
response=client.describe_auto_scaling_groups()

for i in response['AutoScalingGroups']:
    subnetsIDs[i['VPCZoneIdentifier']]=1


for i in ec2.instances.all():
    subnetsIDs[i.subnet_id]=1
    
for each in subnetsIDs.keys():
    if subnetsIDs[each] == 0:
        print(each)