code: def posterize(image, bits):
      """
      Reduce the number of bits for each color channel.
  
      :param image: The image to posterize.
      :param bits: The number of bits to keep for each channel (1-8).
      :return: An image.
      """
      lut = []
      mask = ~(2 ** (8 - bits) - 1)
      for i in range(256):
          lut.append(i & mask)
      return _lut(image, lut)
  
imports: import functools
  import operator
  from .None import Image
  from ._util import isStringType
label_desc: Use image node_id 17 for calling a global function with example usage: posterize(image, bits) and returns: _lut