code: def is_valid(self):
          
  
          valid = True
          if not self.sons:
              valid = False
          else:
              for son in self.sons:
                  if isinstance(son, DependencyNode) and not son.is_valid():
                      self.configuration_errors.extend(son.configuration_errors)
                      valid = False
          return valid
docs: Check if all leaves are correct (no error)
  
          :return: True if correct, otherwise False
          :rtype: bool