Suppose you have multiple GPU devices.

To specify a GPU to use (or specify no GPU to use), changing the CUDA_VISIBLE_DEVICES is dangerous, since it’s an environment variable, changing which causes side-effect. A better way is to set (suppose we are to use \device:GPU:1)

config = tf.ConfigProto()
config.gpu_options.visible_device_list = '1'
sess = tf.Session(config=config)

with sess:
    ...  # the rest of your code.

This will not change the environment variable (tested), thus safe.