Cannot set value 'RGBA' to color_mode in Blender by script (Python)

Question

I use script to split render scene to smaller pieces (tiles) - by setting use_border = True. Then "tiles" are rendered, and after process finishing, I join "tiles" to final image. To make it easier, I use RGBA setting during render process. It works fine. But sometimes (for some input files) I get error: bpy_struct: item.attr = val: enum "RGBA" not found in ('BW', 'RGB')

I set value RGBA by command: bpy.context.scene.render.image_settings.color_mode ='RGBA'

I get the error message during rendering demo file "Barcelona Pavillion" from this web page: www.blender.org/download/demo-files/

In Blender GUI I can change to RGBA mode for this file - even if I set border (by Ctrl+B) and render part of scene.

Do you have any idea how to set RGBA for this file by Python script ?

I use Blender version 2.79b

Runnable example:

import bpy

bpy.ops.wm.open_mainfile (filepath="pavillon_barcelone_v1.2.blend", load_ui=False)

rndr = bpy.context.scene.render

rndr.use_border = True
rndr.use_crop_to_border = False

rndr.border_min_x = 0.3333
rndr.border_min_y = 0.3333
rndr.border_max_x = 0.6666
rndr.border_max_y = 0.6666
bpy.context.scene.cycles.device = 'CPU'

rndr.image_settings.color_mode ='RGBA'

result = bpy.ops.render.render (write_still=True)

Answer 1

Little mistake :-) I haven't set output format. There was set JPEG and it has no transparency.

When I put this line (before RGBA assignment):

rndr.image_settings.file_format = 'PNG'

script works.