Read images in mode-P instead of mode-L

Hi all,
I have a dataset of masks for semantic segmentation.
The masks are .png files, and each pixel of them can have 3 values [1, 2, 255]. The native mode of the masks’ files is mode-P.
Once I read these files with Fastai, I noted that resulted 4 values [0, 76, 242, 255].

I suspect that it is due to the fact that PIL read files in mode-L instead of mode-P.
As suggested here, I tried to override the specific class by:
class PILMask(PILBase): _open_args,_show_args = {'mode':'P'},{'alpha':0.5, 'cmap':'tab20'}
but it does not seem to work with DataBlock(...).

Solved, mapping by a homemade function pixels into the range [0, 1, 2]