code: def is_valid(self, max_age=None):
          ''' Determines if the cache files have expired, or if it is still valid '''
  
          if max_age is None:
              max_age = self.cache_max_age
  
          if os.path.isfile(self.cache_path_cache):
              mod_time = os.path.getmtime(self.cache_path_cache)
              current_time = time()
              if (mod_time + max_age) > current_time:
                  return True
  
          return Falsf is_valid(self, max_age=None):
          ''' Determines if the cache files have expired, or if it is still valid '''
  
          if max_age is None:
              max_age = self.cache_max_age
  
          if os.path.isfile(self.cache_path_cache):
              mod_time = os.path.getmtime(self.cache_path_cache)
              current_time = time()
              if (mod_time + max_age) > current_time:
                  return True
  
          return False
docs: Determines if the cache files have expired, or if it is still valid