import bpy
cube = bpy.data.objects["Cube"]
bpy.context.scene.objects.active = cube

bpy.ops.object.editmode_toggle()
bpy.ops.mesh.select_all(action='SELECT')
bpy.ops.uv.cube_project()
bpy.ops.object.editmode_toggle()

image = bpy.data.images.new(name="test", width=512, height=512)

bpy.ops.object.bake_image()

image = bpy.data.images.new(name="test", width=512, height=512)

for uv_face in cube.data.uv_textures.active.data:
    uv_face.image = image

bpy.ops.object.bake_image()