PyAudio working, but spits out error messages each time

Question

I'm using PyAudio to record input from the microphone.

Since the audio is recording fine for me, should I try to simply suppress its error messages? Or would there be a way of resolving them?

ALSA lib pcm.c:2212:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2212:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2212:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side
ALSA lib audio/pcm_bluetooth.c:1613:(audioservice_expect) BT_GET_CAPABILITIES failed : Input/output error(5)
ALSA lib audio/pcm_bluetooth.c:1613:(audioservice_expect) BT_GET_CAPABILITIES failed : Input/output error(5)
ALSA lib audio/pcm_bluetooth.c:1613:(audioservice_expect) BT_GET_CAPABILITIES failed : Input/output error(5)
ALSA lib audio/pcm_bluetooth.c:1613:(audioservice_expect) BT_GET_CAPABILITIES failed : Input/output error(5)
ALSA lib pcm_dmix.c:957:(snd_pcm_dmix_open) The dmix plugin supports only playback stream
ALSA lib pcm_dmix.c:1018:(snd_pcm_dmix_open) unable to open slave
Cannot connect to server socket err = No such file or directory
Cannot connect to server socket
jack server is not running or cannot be started

Answer 1

You can try to clean up your ALSA configuration, for example,

ALSA lib pcm.c:2212:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2212:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2212:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side

are caused by /usr/share/alsa/alsa.conf:

pcm.rear cards.pcm.rear
pcm.center_lfe cards.pcm.center_lfe
pcm.side cards.pcm.side

Once you comment out these lines, those error message will be gone. You may also want to check ~/.asoundrc and /etc/asound.conf.

That's said, some of those messages are telling something is wrong in your configuration, though they do not cause any real problem. I do not recommend you clean up the alsa.conf, because it's from ALSA originally, it may be overwritten when you update alsa-lib.

There is a way to suppress the message in Python, here is a sample code:

#!/usr/bin/env python
from ctypes import *
import pyaudio

# From alsa-lib Git 3fd4ab9be0db7c7430ebd258f2717a976381715d
# $ grep -rn snd_lib_error_handler_t
# include/error.h:59:typedef void (*snd_lib_error_handler_t)(const char *file, int line, const char *function, int err, const char *fmt, ...) /* __attribute__ ((format (printf, 5, 6))) */;
# Define our error handler type
ERROR_HANDLER_FUNC = CFUNCTYPE(None, c_char_p, c_int, c_char_p, c_int, c_char_p)
def py_error_handler(filename, line, function, err, fmt):
  print 'messages are yummy'
c_error_handler = ERROR_HANDLER_FUNC(py_error_handler)

asound = cdll.LoadLibrary('libasound.so')
# Set error handler
asound.snd_lib_error_set_handler(c_error_handler)
# Initialize PyAudio
p = pyaudio.PyAudio()
p.terminate()

print '-'*40
# Reset to default error handler
asound.snd_lib_error_set_handler(None)
# Re-initialize
p = pyaudio.PyAudio()
p.terminate()

An output from my computer:

messages are yummy
messages are yummy
messages are yummy
messages are yummy
messages are yummy
messages are yummy
----------------------------------------
ALSA lib pcm_dmix.c:1018:(snd_pcm_dmix_open) unable to open slave
ALSA lib pcm.c:2217:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2217:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2217:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side
ALSA lib pcm_dmix.c:957:(snd_pcm_dmix_open) The dmix plugin supports only playback stream
ALSA lib pcm_dmix.c:1018:(snd_pcm_dmix_open) unable to open slave

Those messages are printed out by alsa-lib, not PyAudio or PortAudio. The code directly uses alsa-lib snd_lib_error_set_handler function to set an error handler py_error_handler, which you can use it to drop any message.

I have checked other Python ALSA bindings, pyalsa and PyAlsaAudio, they do not support setting error handler. However, there is an issue on PortAudio, all ALSA error messages seemed to be suppressed before.

Answer 2

All of the above is true and a good solution. I just came here to suggest a nicer way of re-using the error handler code:

from ctypes import *
from contextlib import contextmanager
import pyaudio

ERROR_HANDLER_FUNC = CFUNCTYPE(None, c_char_p, c_int, c_char_p, c_int, c_char_p)

def py_error_handler(filename, line, function, err, fmt):
    pass

c_error_handler = ERROR_HANDLER_FUNC(py_error_handler)

@contextmanager
def noalsaerr():
    asound = cdll.LoadLibrary('libasound.so')
    asound.snd_lib_error_set_handler(c_error_handler)
    yield
    asound.snd_lib_error_set_handler(None)

After doing this you can re-use the error handler by using the noalsaerr context:

with noalsaerr():
    p = pyaudio.PyAudio()
stream = p.open(format=pyaudio.paFloat32, channels=1, rate=44100, output=1)
...

Answer 3

The sounddevice module will suppress those messages, see https://github.com/spatialaudio/python-sounddevice/issues/11.