Stitch multiple images horizontally and vertically to create grid, filling empy grid cells with black in Python

Question

I have 12 images that I want to make a 4x3 stitched image out of. I have used np.concatenate to stitch them together, but is there a better way?

Here is my code:

for exr in range(len(exr_files)):
    if exr == 0:
       stitched_exrs = exr_files_list[exr]
    else:
       stitched_exrs = np.concatenate((stitched_exrs, exr_files_list[exr]), axis = 1)