How can the terminal output of executables run by Python functions be silenced in a general way?

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For the ALSA errors in particular, you can use ALSA’s snd_lib_error_set_handler function as described for example in this question.

For one of my projects, I created a module called mute_alsa, which looks like this:

import ctypes

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


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

c_error_handler = ERROR_HANDLER_FUNC(py_error_handler)

try:
    asound = ctypes.cdll.LoadLibrary('libasound.so.2')
    asound.snd_lib_error_set_handler(c_error_handler)
except OSError:
    pass

And you use it simply by putting the following into your code:

import mute_alsa

In a more general sense, you can prevent anything from printing to
stderr by simply closing the associated file descriptor. For
example, if we try to rm a file that doesn’t exist, we get an error
message printed to stderr:

>>> import sys
>>> import os
>>> os.system('rm /doesnotexist')
rm: cannot remove ‘/doesnotexist’: Is a directory
256

If we close the stderr file descriptor:

>>> os.close(sys.stderr.fileno())

We no longer see any error messages:

>>> os.system('rm /doesnotexist')

The downside to this approach is that it silences all messages to
stderr, which means you will no longer see legitimate error
messages. This may lead to surprising failure modes (the program
exited without printing any errors!).

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