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As it states in the apply()/apply_async() documentation, each call to apply() or apply_async() is equivalent to one call to the function you pass as the first argument (i.e. run_ffprobe).

In your synchronous code, you are passing each element of args_list to a separate run_ffprobe() call:

I solved this by the accepted answer in my related question.

I had forgotten about the return carriage at the end of each line. Solutions given are as follows:

1. Use universal_newlines=True in the subprocess call.
   • stderr = Popen(("ffprobe", file_path), shell=True, stderr=PIPE, universal_newlines=True).communicate()[1]

2. Stripping the whitespace around the line from stderr.

   • *.communicate()[1].decode().rstrip() to strip all whitespace at the end.
   • *.communicate()[1].decode().strip() to strip all wightspace around.
   • *.communicate()[1].decode()[:-2] to remove the last two characters.

3. Swallowing \r in the regex pattern.

   • findall(r"(\w+)\s+:\s(.+)\r$", stderr, MULTILINE)

This is all very helpful, however I used none of these suggestions.

I didn't know that FFPROBE offers JSON output to STDOUT, but it does. The code to do that is below.