xscreensaver rotate image in sequence

xscreensaver-getimage-file

#!/usr/bin/env python3

import sys
import os
import time
import fcntl

def main(image_dir):
    files = []
    for (dirpath, dirnames, filenames) in os.walk(image_dir):
        for filename in filenames:
            files.append(os.path.join(dirpath, filename))
    sortedfiles = sorted(files, key=lambda s: s.lower())

    home_dir = os.environ['HOME']
    cache_filename = os.path.join(home_dir, '.cache', 'xscreensaver', 'xscreensaver-getimage-alt.cache')
    os.makedirs(os.path.dirname(cache_filename), exist_ok=True)
    cache_fd = os.open(cache_filename, os.O_CREAT | os.O_RDWR, 0o666)
    fcntl.lockf(cache_fd, fcntl.LOCK_EX)

    cache_file = os.fdopen(cache_fd, 'r+', closefd=False)
    line = cache_file.readline()
    if not line:
        if not sortedfiles:
            sys.exit(1);
        else:
            newfile = sortedfiles[0]
            write_cache(cache_file, newfile)
            print(newfile)
    else:
        updatetime_filename = line.split(' ', 1)
        updatetime = float(updatetime_filename[0])
        filename = updatetime_filename[1]

        currenttime = time.time()
        if currenttime - updatetime > 2:
            # get next item in list
            if filename in sortedfiles:
                newindex = (sortedfiles.index(filename) + 1) % len(sortedfiles)
            else:
                newindex = 0
            newfile = sortedfiles[newindex]
            write_cache(cache_file, newfile)
            print(newfile)
        else:
            print(filename)

    fcntl.lockf(cache_fd, fcntl.LOCK_UN)
    os.close(cache_fd)

def write_cache(cache_file, filename):
    cache_file.seek(0)
    cache_file.truncate(0)
    cache_file.write(str(time.time()))
    cache_file.write(' ')
    cache_file.write(filename)
    cache_file.flush()

if __name__ == "__main__":
   main(sys.argv[-1])