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

I have Keras installed with the Tensorflow backend and CUDA. I'd like to sometimes on demand force Keras to use CPU. Can this be done without say installing a separate CPU-only Tensorflow in a virtual environment? If so how? If the backend were Theano, the flags could be set, but I have not heard of Tensorflow flags accessible via Keras.

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

From: stackoverflow.com/q/40690598