The tf.device() argument selects the device (CPU or GPU). The with block ensures the operations for which the device is selected. All the variables, constants, and operations defined within the with block will use the device selected in tf.device(). Session configuration is controlled using tf.ConfigProto. By setting the allow_soft_placement and log_device_placement flags, we tell TensorFlow to automatically choose the available devices in case the specified device is not available, and to give log messages as output describing the allocation of devices while the session is executed.