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ElectronicsGeek
Power User (Posts: 359)

Pipes in Subprocess [Python]
06 May 2014 12:36:10 pm

I've been writing a little program for a radio transmitter I built with the Raspberry Pi. Everything had been working fine up until I used subprocess for the handling of the command line utilities needed to run the said program.

The question I pose to everyone here is how should I go about converting the below command (which uses pipes) into code that utilises 'subprocess.call()'

Code:

ffmpeg -i input.mp3 -f s16le -ar 22.05k -ac 1 - | sudo ./pifm -

Many thanks!

elfprince13
2
OVER NINE THOUSAND! (Posts: 11872)

06 May 2014 01:17:39 pm

Code:

ffmpeg = Popen(["ffmpeg","-i","input.mp3","-f","s161e","-ar","22.05k","-ac","1","-",stdout=subprocess.PIPE)


sp = Popen(["sudo","./pifm","-"],stdin=ffmpeg.stdout)

Do what you want with the output/return code by checking them.

ElectronicsGeek
Power User (Posts: 359)

06 May 2014 01:25:30 pm

Cheers, elfprince, that's great! I assume it will work the same with subprocess.call()?

elfprince13
2
OVER NINE THOUSAND! (Posts: 11872)

06 May 2014 01:57:30 pm

The second one can use subprocess.call, but you need to have a Popen handle to ffmpeg to get the pipe from.

I recommend following that with sp.wait() which has the same return value as the call would have.

ElectronicsGeek
Power User (Posts: 359)

06 May 2014 02:04:34 pm

I assume I'd also be best to kill the main process too, when it's done.

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