In Python, how do I draw/ save a monochrome 2 bit BMP with Wand

I have a need to create a 2 bit monochrome Windows BMP format image, and need to draw lines in a pattern. Since I have just started using python, I followed a tutorial and installed the Wand module.

Drawing is fine, I get the content I need.

Problem is saving the image. No matter what I do, the resulting image is always 24 bpp BMP.

What I have so far:

from wand.image import Image
from wand.drawing import Drawing
from wand.color import Color
draw=Drawing()

### SET VARS
mystepvalue = 15

with Image(filename='empty.bmp') as image:
          print(image.size)
          image.antialias = False
          draw.stroke_width = 1
          draw.stroke_color = Color('black')
          draw.fill_color = Color('black')
          #
          # --- draw the lines ---
          #
          for xpos in range(0,4790,mystepvalue):
                    draw.line( (xpos,0), (xpos, image.height) )

          draw(image)  # Have to remember this to see anything...

          # None of this makes a 2bit BMP
          image.depth = 2
          image.colorspace = "gray"
          image.antialias = False
          image.monochrome = True
          image.imagedepth =2
          image.save(filename='result.bmp')

Source image is 4800 x 283 px 169kb, result.bmp is same, but 4.075 kb.

It's probably the order of the color settings that I miss.

Try using Image.quantize() to remove all the unique gray colors before setting the depth/colorspace properties.

    # ...
    image.quantize(2, colorspace_type='gray', dither=True)
    image.depth = 2
    image.colorspace = 'gray'
    image.type = 'bilevel'  # or grayscale would also work.
    image.save(filename='result.bmp')

Also remember filename='bmp3:result.bmp' or filename='bmp2:result.bmp' controls which BMP version to encode with.