code: def _custom_policy(resource, expires=None, valid_after=None, ip_address=None):
          
          condition = {}
          if expires:
              condition["DateLessThan"] = {"AWS:EpochTime": expires}
          if valid_after:
              condition["DateGreaterThan"] = {"AWS:EpochTime": valid_after}
          if ip_address:
              if '/' not in ip_address:
                  ip_address += "/32"
              condition["IpAddress"] = {"AWS:SourceIp": ip_address}
          policy = {"Statement": [{
                       "Resource": resource,
                       "Condition": condition}]}
          return json.dumps(policy, separators=(",", ":"))
docs: Creates a custom policy string based on the supplied parameters.