Can Keras with Tensorflow backend be forced to use CPU or GPU at will?

If you want to force Keras to use CPU

Way 1

import os
os.environ["CUDA_DEVICE_ORDER"] = "PCI_BUS_ID"   # see issue #152
os.environ["CUDA_VISIBLE_DEVICES"] = ""

before Keras / Tensorflow is imported.

Way 2

Run your script as

$ CUDA_VISIBLE_DEVICES="" ./your_keras_code.py

See also

1. https://github.com/keras-team/keras/issues/152
2. https://github.com/fchollet/keras/issues/4613