TypeError: export_png()恰好接受3个参数(给定4个)

Question

当我尝试通过GIMP的gui运行以下python脚本时：

#!/usr/bin/env python

from gimpfu import *

def export_png(image, drawable, self):
    filename=image.name
    interlace=0
    compression=0
    bkgd=0
    gama=0
    off=0
    phys=0
    time=0

    pdb.file_png_save(image, drawable, filename, raw_filename, interlace, compression, bkgd, gama, offs, phys, time)

register(
        "python_fu_export_png",
        "Exports the current image as png",
        "Exports the current image as png",
        "HHP",
        "HHP",
        "2017",
        "<Image>/Image/Export as png",
        "*",
        [
        (PF_IMAGE, "image", "takes current image", None),
        (PF_DRAWABLE,"drawable", "input layer",None),
        ],
        [],
        export_png
        )

main()

我得到了这个错误：

Traceback (most recent call last):
  File "/Applications/GIMP.app/Contents/Resources/lib/gimp/2.0/python/gimpfu.py", line 736, in response
    dialog.res = run_script(params)
  File "/Applications/GIMP.app/Contents/Resources/lib/gimp/2.0/python/gimpfu.py", line 361, in run_script
    return apply(function, params)
TypeError: export_png() takes exactly 3 arguments (4 given)

为什么会这样呢？我以为我只是把PF_Image和PF_DRAWABLE传递给函数，为什么它说我给了四个参数呢？