render.py

# Mark all scene devices as GPU for cycles

bpy.context.scene.cycles.device = 'GPU'

for scene in bpy.data.scenes:
    scene.cycles.device = 'GPU'

# Enable CUDA

bpy.context.user_preferences.addons['cycles'].preferences.compute_device_type = 'CUDA'

# Enable and list all devices, or optionally disable CPU
for d in bpy.context.user_preferences.addons['cycles'].preferences.devices:
    d.use = True
    # if d.type == 'CPU':
    #     d.use = False
    print("Device '{}' type {} : {}" . format(d.name, d.type, d.use))

# Optionally set a filepath override, if you haven't already in your file or via adjust.py
# bpy.data.scenes["Scene"].render.filepath = "/tmp/output.png"

# write_still will save the rendered image to the given output path; not necessary for node based setups
bpy.ops.render.render(write_still=True)