subprocess "TypeError: a bytes-like object is required, not 'str'"

I'm using this code from a previously asked question a few years ago, however, I believe this is outdated. Trying to run the code, I receive the error above. I'm still a novice in Python, so I could not get much clarification from similar questions. Why is this happening?

import subprocess

def getLength(filename):
  result = subprocess.Popen(["ffprobe", filename],
    stdout = subprocess.PIPE, stderr = subprocess.STDOUT)
  return [x for x in result.stdout.readlines() if "Duration" in x]

print(getLength('bell.mp4'))

Traceback

Traceback (most recent call last):
  File "B:\Program Files\ffmpeg\bin\test3.py", line 7, in <module>
    print(getLength('bell.mp4'))
  File "B:\Program Files\ffmpeg\bin\test3.py", line 6, in getLength
    return [x for x in result.stdout.readlines() if "Duration" in x]
  File "B:\Program Files\ffmpeg\bin\test3.py", line 6, in <listcomp>
    return [x for x in result.stdout.readlines() if "Duration" in x]
TypeError: a bytes-like object is required, not 'str'

Solution

subprocess returns bytes objects for stdout or stderr streams by default. That means you also need to use bytes objects in operations against these objects. "Duration" in x uses str object. Use a bytes literal (note the b prefix):

return [x for x in result.stdout.readlines() if b"Duration" in x]

or decode your data first, if you know the encoding used (usually, the locale default, but you could set LC_ALL or more specific locale environment variables for the subprocess):

return [x for x in result.stdout.read().decode(encoding).splitlines(True)
        if "Duration" in x]

The alternative is to tell subprocess.Popen() to decode the data to Unicode strings by setting the encoding argument to a suitable codec:

result = subprocess.Popen(
    ["ffprobe", filename],
    stdout=subprocess.PIPE, stderr = subprocess.STDOUT,
    encoding='utf8'
)

If you set text=True (Python 3.7 and up, in previous versions this version is called universal_newlines) you also enable decoding, using your system default codec, the same one that is used for open() calls. In this mode, the pipes are line buffered by default.