getregions.py

import json, botocore
from boto3.session import Session

def serialize_sets(obj):
  if isinstance(obj, set):
    return list(obj)
  return obj

s = Session()
regions = s.get_available_regions('lambda')
enabled_regions = set()
for region in regions:
  sts_client = s.client('sts', region_name=region)
  try:
    sts_client.get_caller_identity()
    enabled_regions.add(region)
  except botocore.exceptions.ClientError as e:
    if e.response['Error']['Code'] == "InvalidClientTokenId":
      pass
    else:
      raise
print(json.dumps(enabled_regions, default=serialize_sets))