code: def verify_integrity(self):
          
          if not self.__integrity_check:
              if not self.__appid:
                  raise Exception('U2F_APPID was not defined! Please define it in configuration file.')
  
              if self.__facets_enabled and not len(self.__facets_list):
                  raise Exception("""U2F facets been enabled, but U2F facet list is empty.
                                     Please either disable facets by setting U2F_FACETS_ENABLED to False.
                                     Or add facets list using, by assigning it to U2F_FACETS_LIST.
                                  """)
  
              # Injection
              
              undefined_message = 'U2F {name} handler is not defined! Please import {name} through {method}!'
  
              if not self.__get_u2f_devices:
                  raise Exception(undefined_message.format(name='Read', method='@u2f.read'))
  
              if not self.__save_u2f_devices:
                  raise Exception(undefined_message.format(name='Save', method='@u2f.save'))
  
  
              if not self.__call_success_enroll:
                  raise Exception(undefined_message.format(name='enroll onSuccess', method='@u2f.enroll_on_success'))
  
              if not self.__call_success_sign:
                  raise Exception(undefined_message.format(name='sign onSuccess', method='@u2f.sign_on_success'))
  
              self.__integrity_check = True
  
          return True
docs: Verifies that all required functions been injected.