To ensure that a GPU version TensorFlow process only runs on CPU:

import os
os.environ["CUDA_VISIBLE_DEVICES"]="-1"    
import tensorflow as tf

For more information on the CUDA_VISIBLE_DEVICES, have a look to this answer or to the CUDA documentation.

import tensorflow as tf
sess = tf.Session(config=tf.ConfigProto(device_count={'GPU': 0}))

Bear in mind that this method prevents the TensorFlow Graph from using the GPU but TensorFlow still lock the GPU device as described in this an issue opened on this method. Using the CUDA_VISIBLE_DEVICES seems to be the best way to ensure that TensorFlow is kept away from the GPU card (see this answer).

To use a particular set of GPU devices, the CUDA_VISIBLE_DEVICES environment variable can be used:

import os
os.environ["CUDA_DEVICE_ORDER"]="PCI_BUS_ID" # see issue #152
os.environ["CUDA_VISIBLE_DEVICES"]="0" # Will use only the first GPU device


os.environ["CUDA_VISIBLE_DEVICES"]="0,3" # Will use only the first and the fourth GPU devices

(Quoted from this answer; more information on the CUDA environment variables here.)

from tensorflow.python.client import device_lib
print(device_lib.list_local_devices())

By default, TensorFlow pre-allocate the whole memory of the GPU card (which can causes CUDA_OUT_OF_MEMORY warning).

To change this, it is possible to

• change the percentage of memory pre-allocated, using per_process_gpu_memory_fraction config option,

  A value between 0 and 1 that indicates what fraction of the
  available GPU memory to pre-allocate for each process. 1 means
  to pre-allocate all of the GPU memory, 0.5 means the process
  allocates ~50% of the available GPU memory.

• disable the pre-allocation, using allow_growth config option. Memory allocation will grow as usage grows.

  If true, the allocator does not pre-allocate the entire specified
  GPU memory region, instead starting small and growing as needed.

For example:

config = tf.ConfigProto()
config.gpu_options.per_process_gpu_memory_fraction = 0.4
sess = tf.Session(config=config) as sess:

or

config = tf.ConfigProto()
config.gpu_options.allow_growth = True
sess= tf.Session(config=config):

More information on the config options here.