Python Pillow Image Invert

# -*- encoding:utf-8 -*-
import os
import sys
from PIL import Image
from PIL import ImageOps
def img_gray_invert(img_path):
  """
  invert input image.
  """
  if not os.path.isfile(img_path):
    print "Error for input file path."
    return
  image = Image.open(img_path)
  image = image.convert("L")
  inverted_image = ImageOps.invert(image)
  return inverted_image
if __name__ == '__main__':
  argv = sys.argv
  if len(argv) != 3:
    print """Example:
    python gray_invert.py test/htc.png test/htc_inv.png
    """
  else:
    img_file_path = argv[1]
    invert_image = img_gray_invert(img_file_path)
    img_file_out = argv[2]
    invert_image.save(img_file_out)