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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update_endpoint(self, endpoint_name, endpoint_config_name):
 """ Update an Amazon SageMaker ``Endpoint`` according to the endpoint configuration specified in the request Raise an error if endpoint with endpoint_name does not exist. Args: endpoint_name (str):
 Name of the Amazon SageMaker ``Endpoint`` to update. endpoint_config_name (str):
 Name of the Amazon SageMaker endpoint configuration to deploy. Returns: str: Name of the Amazon SageMaker ``Endpoint`` being updated. """

        if not _deployment_entity_exists(lambda: self.sagemaker_client.describe_endpoint(EndpointName=endpoint_name)):
            raise ValueError('Endpoint with name "{}" does not exist; please use an existing endpoint name'
                             .format(endpoint_name))

        self.sagemaker_client.update_endpoint(EndpointName=endpoint_name,
                                              EndpointConfigName=endpoint_config_name)
        return endpoint_name