code: def authenticate(self, environ):
          
          try:
              hd = parse_dict_header(environ['HTTP_AUTHORIZATION'])
          except (KeyError, ValueError):
              return False
  
          return self.credentials_valid(
              hd['response'],
              environ['REQUEST_METHOD'],
              environ['httpauth.uri'],
              hd['nonce'],
              hd['Digest username'],
          )
docs: Returns True if the credentials passed in the Authorization header are
          valid, False otherwise.