Python ffmpeg displaying output

I want to run command

ffmpeg -i movie.mp4 -vf scale=224:224 movie_224.mp4

to resize video using python language.

I have code for it:

import subprocess
sys_cmd = ["ffmpeg", "-i", "movie.mp4", "-vf", "scale=224:224", movie_224.mp4]
subprocess.check_call(sys_cmd, stdout=subprocess.DEVNULL)

I don't know why I am getting std output even if I have given the argument subprocess.DEVNULL.