human
Solve the following python programming problem given the following code:
@ cached_property 
def server(self) : 
    '
        All in one endpoints. This property is created automatically
        if you have implemented all the getters and setters.

        However, if you are not satisfied with the getter and setter,
        you can create a validator with :class:`OAuth2RequestValidator`::

            class MyValidator(OAuth2RequestValidator):
                def validate_client_id(self, client_id):
                    # do something
                    return True

        And assign the validator for the provider::

            oauth._validator = MyValidator()
        ' 
    expires_in = self.app.config.get ('OAUTH2_PROVIDER_TOKEN_EXPIRES_IN') 
    token_generator = self.app.config.get ('OAUTH2_PROVIDER_TOKEN_GENERATOR', None) 
    if (token_generator and (not callable (token_generator))) : 
        token_generator = import_string (self) 
refresh_token_generator = self.app.config.get ('OAUTH2_PROVIDER_REFRESH_TOKEN_GENERATOR', None) 
    if (refresh_token_generator and (not callable (refresh_token_generator))) : 
        refresh_token_generator = import_string (refresh_token_generator) 
if hasattr (self, '_validator') : 
        return Server (self._validator, token_expires_in = expires_in, token_generator = token_generator, refresh_token_generator = refresh_token_generator) 
if (hasattr (self, '_clientgetter') and hasattr (self, '_tokengetter') and hasattr (self, '_tokensetter') and hasattr (self, '_grantgetter') and hasattr (self, '_grantsetter')) : 
        usergetter = None 
        if hasattr (self, '_usergetter') : 
            usergetter = self._usergetter 
validator = OAuth2RequestValidator (clientgetter = self._clientgetter, tokengetter = self._tokengetter, grantgetter = self._grantgetter, usergetter = usergetter, tokensetter = self._tokensetter, grantsetter = self._grantsetter) 
        self._validator = validator 
        return Server (validator, token_expires_in = expires_in, token_generator = token_generator, refresh_token_generator = refresh_token_generator)...

    
    

    Fix the buggy line: token_generator = import_string (self)
assistant
#Here is Python program to fix the buggy line: token_generator = import_string (self)
.

token_generator = import_string (token_generator)