Converting videos using python and FFMPEG

ffmpeg-convert.py

import os, subprocess

inDir = 'ka-lite-en/content/'
outDir = 'ka-lite-en-converted/content/'
ffmpeg = [
    "ffmpeg", 
    "-i", 
    "in.mp4", 
    "-r", 
    "15", 
    "-vcodec", 
    "libx264", 
    "-crf", 
    "32", 
    "-preset", 
    "veryfast", 
    "-profile:v", 
    "baseline",
    "-level", 
    "3", 
    "-refs", 
    "6", 
    "-vf", 
    "scale=720:-2,format=yuv420p",
    "-acodec", 
    "copy", 
    "output.mp4", 
    "-y"
]

with open('progress.log', 'a') as log:
    count = 0
    for fn in os.listdir(inDir):
        count = count + 1
        ext = fn.split('.')
        if (ext and len(ext) > 1 and ext[1] == 'mp4'):
            filename = ext[0] + '.' +ext[1]
            ffmpeg[2] = inDir + filename
            ffmpeg[-2] = outDir + filename
            subprocess.call(ffmpeg)
            print ('converted ' + filename)
            log.write(filename + ' converted. ' + str(count) + '\n')
    log.close()