When you use subprocess, your command must either be a string that looks exactly like what you would type on the command line (and you set shell=True), or a list where each command is an item in the list (and you take the default shell=False). In either case, you have to deal with the variable part of the string. For instance, the operating system has no idea what "%03d" is, you have to fill it in.

I can't tell from your question exactly what the parameters are, but lets assume you want to convert frame 3, it would look something like this in a string:

my_frame = 3
subprocess.call(
    'ffmpeg -r 10 -i frame%03d.png -r ntsc movie%03d.mpg' % (my_frame, my_frame),
    shell=True)

Its kinda subtle in this example, but that's risky. Suppose these things were in a directory whose name name had spaces (e.g., ./My Movies/Scary Movie). The shell would be confused by those spaces.

So, you can put it into a list and avoid the problem

my_frame = 3
subprocess.call([
    'ffmpeg',
    '-r', '10',
    '-i', 'frame%03d.png' % my_frame,
    '-r', 'ntsc',
    'movie%03d.mpg' % my_frame,
])

More typing, but safer.

In this specific case for capturing ffmpeg's status output (which goes to STDERR), this SO question solved it for me: FFMPEG and Pythons subprocess

The trick is to add universal_newlines=True to the subprocess.Popen() call, because ffmpeg's output is in fact unbuffered but comes with newline-characters.

cmd = "ffmpeg -i in.mp4 -y out.avi"
process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,universal_newlines=True)
for line in process.stdout:
    print(line)

Also note that in this code sample the STDERR status output is directly redirected to subprocess.STDOUT

I would like to understand how to read the following ffmpeg instruction

Your ffmpeg command is semi-obfuscated by scripting so the actual command is not known, but here's an explanation of each option:

• -i indicates the input.
• -r 1 sets output frame rate to 1. This is not needed if you want to output a single image or if you want to output all images. In this example it is used to output one frame per second which would skip many frames.
• -s qvga sets output width x height to "qvga" which is an alias for 320x240.
• -t 1 sets the output duration to 1 second. This is not needed if you want to output a single image or if you want to output all images. It is often added by rookie users trying to output a single image but -frames:v 1 should be used instead.
• -f image2 An often superfluous option used to set the output format or muxer. It is used if your output name is ambiguous (perhaps due to scripting). Otherwise, ffmpeg will automatically choose the proper muxer for image outputs.

how can l adapt it to get all the frames of a given video ?

The simplest, unscripted command to get all of the frames is:

ffmpeg -i input %04d.png

This will output 0001.png, 0002.png, 0003.png, etc. If you want more than a numerical sequence you can use something like output_%05d.png which would result in output_00001.png.

For more info see FFmpeg Documentation: Image Muxer.