Trying to chain processes together on a pipeline

[Andrew Berg]

I'm working on an audio/video converter script (moving from bash to
Python for some extra functionality), and part of it is chaining the
audio decoder (FFmpeg) either into SoX to change the volume and then to
the Nero AAC encoder or directly into the Nero encoder. This is the
chunk of code from my working bash script to give an idea of what I'm
trying to accomplish (it's indented because it's nested inside a while
loop and an if statement):
>         if [ "$process_audio" = "true" ]
>         then
>             if [ $vol == 1.0 ]
>             then
>                 ffmpeg -i "${ifile_a}" -f wav - 2>$nul | neroaacenc
> -ignorelength -q 0.4 -if - -of ${prefix}${zero}${ep}.m4a
>             else
>                 # the pipeline-as-file option of sox fails on Windows
> 7, so I use the safe method since there's only one pipeline going into sox
>                 ffmpeg -i "${ifile_a}" -f sox - 2>$nul | sox -t sox -
> -t wav - vol $vol 2>$nul | neroaacenc -ignorelength -q 0.4 -if - -of
> ${prefix}${zero}${ep}.m4a
>             fi
>         else
>             echo "Audio skipped."
>         fi
This is pretty easy and straightforward in bash, but not so in Python.
This is what I have in Python (queue[position] points to an object I
create earlier that holds a bunch of info on what needs to be encoded -
input and output file names, command line options for the various
encoders used, and so forth), but clearly it has some problems:
>     try:
>         ffmpeg_proc = subprocess.Popen(queue[position].ffmpeg_cmd,
> stdout=subprocess.PIPE, stderr=os.devnull)
>     except WindowsError:
>         error_info = str(sys.exc_info()[1])
>         last_win_error_num = find_win_error_no(error_msg=error_info)
>         if last_win_error_num == '2': # Error 2 = 'The system cannot
> find the file specified'
>             logger.critical('Could not execute ' +
> queue[position].ffmpeg_exe + ': File not found.')
>         elif last_win_error_num == '193': # Error 193 = '%1 is not a
> valid Win32 application'
>             logger.critical('Could not execute ' +
> queue[position].ffmpeg_exe + ': It\'s not a valid Win32 application.')
>         break
>     if queue[position].vol != 1:
>         try:
>             sox_proc = subprocess.Popen(queue[position].sox_cmd,
> stdin=ffmpeg_proc.stdout, stdout=subprocess.PIPE, stderr=os.devnull)
>         except WindowsError:
>             error_info = str(sys.exc_info()[1])
>             last_win_error_num = find_win_error_no(error_msg=error_info)
>             if last_win_error_num == '2': # Error 2 = 'The system
> cannot find the file specified'
>                 logger.critical('Could not execute ' +
> queue[position].sox_exe + ': File not found.')
>             elif last_win_error_num == '193': # Error 193 = '%1 is not
> a valid Win32 application'
>                 logger.critical('Could not execute ' +
> queue[position].sox_exe + ': It\'s not a valid Win32 application.')
>             break
>         wav_pipe = sox_proc.stdout
>     else:
>         wav_pipe = ffmpeg_proc.stdout
>     try:
>         nero_aac_proc = subprocess.Popen(queue[position].nero_aac_cmd,
> stdin=wav_pipe)
>     except WindowsError:
>         error_info = str(sys.exc_info()[1])
>         last_win_error_num = find_win_error_no(error_msg=error_info)
>         if last_win_error_num == '2': # Error 2 = 'The system cannot
> find the file specified'
>             logger.critical('Could not execute ' +
> queue[position].sox_exe + ': File not found.')
>         elif last_win_error_num == '193': # Error 193 = '%1 is not a
> valid Win32 application'
>             logger.critical('Could not execute ' +
> queue[position].sox_exe + ': It\'s not a valid Win32 application.')
>         break
>     
>     ffmpeg_proc.wait()
>     if queue[position].vol != 1:
>         sox_proc.wait()
>     nero_aac_proc.wait()
>     break
Note: those break statements are there to break out of the while loop
this is in.
Firstly, that first assignment to ffmpeg_proc raises an exception:
> Traceback (most recent call last):
>   File "C:\Users\Bahamut\workspace\Disillusion\disillusion.py", line
> 288, in <module>
>     ffmpeg_proc = subprocess.Popen(queue[position].ffmpeg_cmd,
> stdout=subprocess.PIPE, stderr=os.devnull)
>   File "C:\Python32\lib\subprocess.py", line 700, in __init__
>     errread, errwrite) = self._get_handles(stdin, stdout, stderr)
>   File "C:\Python32\lib\subprocess.py", line 861, in _get_handles
>     errwrite = msvcrt.get_osfhandle(stderr.fileno())
> AttributeError: 'str' object has no attribute 'fileno'
I'm not really sure what it's complaining about since the exception
propagates from the msvcrt module through the subprocess module into my
program. I'm thinking it has to do my stderr assignment, but if that's
not right, I don't know what is.
Secondly, there are no Popen.stdout.close() calls because I'm not sure
where to put them.
Thirdly, I have nearly identical except WindowsError: blocks repeated -
I'm sure I can avoid this with decorators as suggested in a recent
thread, but I haven't learned decorators yet. This code isn't very
complete; I'm trying to get it to work in ideal conditions first, then
worry about how to handle failure.

Using Python 3 on Windows 7.