Python: Send command to mplayer under slave mode

Question

I'm trying to send command via pipe to mplayer when running it under slave mode like this:

import subprocess, time
# start mplayer
song = 'mysong.mp3'
cmd = ['mplayer', '-slave', '-quiet', song]
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stdin=subprocess.PIPE)

# send a command every 3 seconds.
# Full command reference here: http://www.mplayerhq.hu/DOCS/tech/slave.txt 
while True:
    print('sleep 3 seconds ...')
    time.sleep(3)
    cmd = 'get_meta_artist'
    print('send command: {}'.format(cmd))
    p.stdin.write(cmd)
    output = p.communicate()[0]
    print(output)

But the output was nothing.

I took the example from this question.

Running the same mplayer command in the terminal works fine. What am I missing here?

UPDATE:

I changed my cmd from "get_meta_artist" to "get_meta_artist\n" so that a line-break is sent to the pipe as well but still I got nothing in the output.

UPDATE2:

I changed the cmd into "\npause\n" and the music was paused. So that means sending command via stdin worked. It means the output string of "\nget_meta_artist\n" command didn't get piped back as expected....

glglgl · Accepted Answer · 2013-04-07T06:42:07.497

You can only use .communicate() once per subprocess. So using it in a while loop doesn't work.

Instead, you should parse the output of p.stdout directly. There seems to be one line per answer if there is an answer.

In order to prevent blocking, you have 3 options:

Work with threads. You have a separate thread who reads from p.stdout and sends its data to the main thread. It blocks if no data is available.
Set p.stdout to non-blocking mode. Essentially, you'll have to do this:
```
import fcntl, os
fcntl.fcntl(p.stdout.fileno(), fcntl.F_SETFL,
    fcntl.fcntl(p.stdout.fileno(), fcntl.F_GETFL) | os.O_NONBLOCK)
```
If you read then without data being available, you get an exception (IOError: [Errno 11] Resource temporarily unavailable).
Work with select.select(): perform p.stdout.readline() only if select.select([p.stdout], [], [], <timeout>)[0] is a non-empty list. In that case, the given file object is guaranteed to have data available and to not block on reading.

In order to separate "garbage output" from "useful" output, you could do this:

def perform_command(p, cmd, expect):
    import select
    p.stdin.write(cmd + '\n') # there's no need for a \n at the beginning
    while select.select([p.stdout], [], [], 0.05)[0]: # give mplayer time to answer...
        output = p.stdout.readline()
        print("output: {}".format(output.rstrip()))
        split_output = output.split(expect + '=', 1)
        if len(split_output) == 2 and split_output[0] == '': # we have found it
            value = split_output[1]
            return value.rstrip()

and then do

print perform_command(p, 'get_meta_artist', 'ANS_META_ARTIST')
print perform_command(p, 'get_time_pos', 'ANS_TIME_POSITION')

Thanks, that explained why I don't see a recurring behaviour. Could you explain a bit more about "switch ne of the std* objects to non-blocking mode" please?? — kakyo, Apr 06 '13 at 23:25
Thank you so much for the detailed answer. I works perfectly. I just read some material about select and non-blocking socket in network programming, and didn't know how it scales to local process stuff until now. I'll need to learn more about this. — kakyo, Apr 07 '13 at 18:43

score 1 · Answer 2 · answered Apr 07 '13 at 02:47

I'm now doing it this way and I start to get output:

 while True:
    cmd = '\nget_meta_artist\n'
    p.stdin.write(cmd)
    output = p.stdout.readline()
    print("output: {}".format(output.rstrip()))
    sys.stdout.flush()

Although I still need to figure out a way to bypass the first flush of mplayer's own initialization stdout but I consider my problem solved.

Thanks to glglgl for giving me useful hints.

Python: Send command to mplayer under slave mode

2 Answers2

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