Convert e-books from epub to mobi - recursive & parallelized python script

convert.py

#!/usr/bin/python3
import os, time, glob, subprocess

files = glob.iglob('./**/*.epub', recursive=True)

workers = []
while files or workers:
    files = list(files)
    while len(workers) < 4 and files:
        f = files[0]
        files = files[1:]
        mobi = os.path.splitext(f)[0]+'.mobi'
        if not os.path.isfile(mobi):
            w = subprocess.Popen(['ebook-convert', f, mobi])
            workers.append(w)
    for w in list(workers):
        if w.poll() is not None:
            workers.remove(w)
    time.sleep(0.1)