PythonMagick: Convert RGB to Grayscale

PythonMagick: Convert RGB to Grayscale

I found this description link_1, link_2.
Trying to do so
import PythonMagick
img = PythonMagick.Image('file.png')
img.colorSpace('GRAYColorspace')
Return this
ArgumentError: Python argument types in
Image.colorSpace(Image, str)
did not match C++ signature:
colorSpace(class Magick::Image {lvalue})
colorSpace(class Magick::Image {lvalue}, enum MagickCore::ColorspaceType)
How to do it?