How to use the annif.backend.backend function in annif

def learn(self, corpus):
        """further train the project using documents from a metadata source"""

        corpus.set_subject_index(self.subjects)
        if isinstance(
                self.backend,
                annif.backend.backend.AnnifLearningBackend):
            self.backend.learn(corpus, project=self)
        else:
            raise NotSupportedException("Learning not supported by backend",
                                        project_id=self.project_id)

def default_params(self):
        params = backend.AnnifBackend.DEFAULT_PARAMS.copy()
        params.update(mixins.ChunkingBackend.DEFAULT_PARAMS)
        params.update(self.DEFAULT_PARAMS)
        return params

def default_params(self):
        return backend.AnnifBackend.DEFAULT_PARAMS