在python脚本中通过子进程将参数传递给mpg123

Question

I have a Python3 script that responds to input from a rotary dial phone (via GPIO pins on a raspberry pi.) If I dial a 1 , my script uses subprocess to tell mpg123 to play a file called 1.mp3 . 2 plays 2.mp3 , and so on up to 7. This is working:

filename = "/media/"+str(number)+".mp3"
player = subprocess.Popen(["mpg123",  filename, "-q"], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

If I dial 8 , the magic 8-ball, the script should behave differently. I want it to call mpg123 with the "little shuffle" flag, -z . From the command line, I know how to make that call:

 mpg123 -z /media/mp3s/*

I want to use this exact syntax in my python3 script, but it isn't working.

 if number ===8:
         player = subprocess.Popen(["mpg123", "-z", "/media/mp3s/*"], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

Nothing happens. Script just waits patiently for the next non-8 input.

As a Plan B, I thought maybe I could pass a list of files to mpg123 with glob.glob :

filelist = glob.glob('/media/mp3s/*.mp3')
print(filelist)
player = subprocess.Popen(["mpg123", "-z", "--list", filelist], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

This crashes the script because TypeError: Can't convert 'list' object to str implicitly (Though it does successfully print out the list of files before it dies.) I think I must be very close with at least one approach, but I can't figure out the last little bit of syntax. Any ideas appreciated.**

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Alright - I figured out a way to make it work, using mpg123's builtin directory specifying flag, -B . I am still curious about how to get one or both of my other approaches to work. For the record, this works:

player = subprocess.Popen(["mpg123", "-Bz", "/media/mp3s/"],.......

Answer 1

To fix approach A, you'll need to set shell=True to get wildcard support. The command itself should then be a string instead of a list however:

if number ===8:
     player = subprocess.Popen("mpg123 -z /media/mp3s/*", stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)

The docs mention this behavior near the bottom of this section:

https://docs.python.org/3.6/library/subprocess.html#frequently-used-arguments

The difference is that without the shell flag, python is running the first entry in the list as the command and using the rest as arguments (the * taken literally). With the shell flag, the entire string is plopped in and the shell is allowed to do what it will. I can't reproduce this exact case but I imagine the command tries to run and fails to find a file called /media/mp3s/* .

I would argue however that your final solution is more pythonic however and you should stick with that. Specifying shell=True generally just means that the python built-ins that accomplish filename expansion, etc. aren't being utilized (examples of these listed in the same doc listed above).

Answer 2

Your solution is probably better, however I thought I would point out the problem with your plan B:

filelist = glob.glob('/media/mp3s/*.mp3')
print(filelist)
player = subprocess.Popen(["mpg123", "-z", "--list", filelist], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

The problem here is you are passing filelist as an item to the list. So you would get a list containing a list:

["mpg123", "-z", "--list", [ '/media/mp3s/1.mp3', '/media/mp3s/2.mp3' ]]

However what you wanted is this:

["mpg123", "-z", "--list", '/media/mp3s/1.mp3', '/media/mp3s/2.mp3']

To get these you need to join the lists together:

player = subprocess.Popen(["mpg123", "-z", "--list"] + filelist, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

Not tested, but I believe this should work.