Fix Halloween photos with Python PIL and multiprocessing

fixphotos.py

#!/usr/bin/env python3

# Fix Halloween photobooth photos
# (Also an excuse to play with thread pools some more)
#
# http://pillow.readthedocs.org/en/latest/index.html
# https://medium.com/@thechriskiehl/parallelism-in-one-line-40e9b2b36148
#
# davep 03-Nov-2014

import sys
import os
import PIL
from PIL import Image
from multiprocessing.dummy import Pool as ThreadPool

def get_file_list(): 
    # get all jpeg files
    return [ f for f in os.listdir(".") if "29_Oct_" in f and f.endswith(".jpg") ]

def fix_filename(filename_in):
    # filename looks like:
    # trunk_or_treat_29_Oct_2014_21_04.jpg
    #
    # The clock on the Pi was wrong. So fix the filename to reflect proper
    # date/time.  29_Oct_2014_22_04 -> 31_Oct_2014_17_04 (best guess subtract
    # five hours from the time)
    fields = filename_in.split("_")

    # ['trunk', 'or', 'treat', '29', 'Oct', '2014', '22', '05.jpg']
    day_idx = 3
    hour_idx = 6

    hour_offset = 5

    new_day = '31'
    new_hour = str(int(fields[hour_idx])-hour_offset)
    fields[day_idx] = new_day
    fields[hour_idx] = new_hour
    new_filename = "_".join(fields)
    return new_filename

def make_thumbnail_filename(filename):
    filename_thumb,ext = os.path.splitext(filename_in)
    filename_thumb += "_thumb.jpg"
    return filename_thumb

def process_image(arg):
    # must rotate the images -π/4 (I rotated them in the Raspberry Pi code for
    # printing) and make a thumbnail
    filename_in, filename_out = arg
    print("{0} -> {1}".format(filename_in,filename_out) )

    img = Image.open(filename_in)
    img.load()

    # rotate
    img_rot = img.rotate(270)
    img_rot.save(filename_out)

    # make thumbnail
    filename_thumb = make_thumbnail_filename(filename_out)
    img_rot.thumbnail((128,128),Image.ANTIALIAS)
    img_rot.save(filename_thumb)

def main() : 
    files_in = get_file_list()

    files_out = [ fix_filename(f) for f in files_in ]
    
    pool = ThreadPool(4)
    results = pool.map(process_image,zip(files_in,files_out))
    pool.close()
    pool.join()

if __name__=='__main__':
    main()