return a list of google compute instances existing before a current run of terraform

terraform_existing_instances.py

import json

data = json.load(open('terraform.tfstate'))
all_res = [mod['resources'].values() for mod in data['modules']]
res_list = reduce(lambda x, y: x + y, all_res)

disk_attrs = [r['primary']['attributes'] for r in res_list if r['type'] == 'google_compute_disk']
print [a['users.0'].split('/')[-1] for a in disk_attrs if 'users.0' in a]