[ Python 常見問題 ] Can Keras with Tensorflow backend be forced to use CPU or GPU at will?

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Question
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.

HowTo
If you want to force Keras to use CPU:

Way1

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1. import os  
2. os.environ["CUDA_DEVICE_ORDER"] = "PCI_BUS_ID"   # see issue #152  
3. 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
https://github.com/keras-team/keras/issues/152
https://github.com/fchollet/keras/issues/4613