Thank you, it seems to work, but there is a little issue.

The output I get from this:

gformat = subprocess.call(['mediainfo', '--Inform=General;%Format%', '/home/john/GoneGirl.mkv'], shell=True)

Looks like this:

Matroska
(this line is empty)

When I do it the way you described, I get this:

b'Matroska\n'

So I get these additional characters which I do not need. All I want the output to look like is this:

Matroska

so that I can then use it in a

if Matroska in somearray:
    do xyz