Question

I have a script that calls ffprobe, parses its output and outputs it to the console.

Here’s a stripped-down version of it without the parsing code and command-line options:

"""Invoke ffprobe to query a video file and parse the output"""

def ffprobe(fname):
    import subprocess as sub
    import re
    p = sub.Popen(['ffprobe', fname], stderr=sub.PIPE)
    stdout, stderr = p.communicate()

def main():
    ffprobe("foo.mp4")
    #print options.formatstr % locals()

if __name__ == '__main__':
    main()

You can see that the only print statement in my code is commented out, so the program shouldn’t really output anything. However, this is what I get:

mpenkov@misha-desktop:~/co/youtube$ python ffprobe.py foo.mp4

mpenkov@misha-desktop:~/co/youtube$ python ffprobe.py foo.mp4

mpenkov@misha-desktop:~/co/youtube$ python ffprobe.py foo.mp4

A newline is mysteriously output by each invocation. Where is it coming from, and how can I deal with it?

There appears to be a similar SO question, except it’s not using the communicate call (http://stackoverflow.com/questions/7985334/python-subprocess-proc-stderr-read-introduce-extra-lines).

Answer 1

I cannot reproduce the problem, so maybe it depends on the file you’re passing to ffprobe.

Anyway, from what I see, stdout isn’t being captured, so maybe the problem is just that ffprobe is printing a new line character to stdout.

To confirm this, please replace:

p = sub.Popen(['ffprobe', fname], stderr=sub.PIPE)
stdin, stderr = p.communicate()

with:

p = sub.Popen(['ffprobe', fname], stdout=sub.PIPE, stderr=sub.PIPE)
stdout, stderr = p.communicate()

In the new version, stdout is captured and the output from p.communicate is correctly named since it returns stdout not stdin.