import bpy

def add_subframe_to_fo_path(scene):
    if scene.name == 'SceneName':
        scene.node_tree.nodes['File Output'].base_path = '//sth/' + str(scene.frame_current) + '-' + str(scene.frame_subframe)


bpy.app.handlers.render_pre.append(add_subframe_to_fo_path)