You can use 

    obj.rotation_euler = Euler((0.3, 0.3, 0.4), 'XYZ')

If all the other track_to options are overkill. For more attribute references see the [Object documentation](http://www.blender.org/api/blender_python_api_2_74_5/bpy.types.Object.html?highlight=rotation_euler#bpy.types.Object.rotation_euler)