Write to a Python subprocess's stdin without communicate()'s blocking behavior

Question

How do I make this a non-blocking call? osd_cat accepts input only as a PIPE which need p.communicate() call making the process to block. Is there any other way to set stdin in Popen?

p = subprocess.Popen(('osd_cat',
                      '-d',
                      '{}'.format(interval)),
                     stdin=subprocess.PIPE)
p.communicate(message)

Use Python Multiprocessing and put your sub task in an other process. Talk to it using a message queue. — Jay M
– Jay M, Commented Jun 26, 2019 at 16:03
Waitaminute, what you're stuck on is how else to feed to stdin? Have you tried p.stdin.write() before asking the question? (Yes, you'll want to do that in a separate thread to prevent blocking in the main process, but this is part of what threads are good for). — Charles Duffy
– Charles Duffy, Commented Jun 26, 2019 at 16:50

Mad Physicist · Accepted Answer · 2019-06-26 16:13:16Z

3

The p.communicate method is a one-shot deal in terms of sending data to the process.

Instead, write directly to p.stdin. If you want to get output, you can read lines from p.stdout. Make sure you pass stdout=subprocess.PIPE to the constructor before attempting to read.

answered Jun 26, 2019 at 16:13

Mad Physicist

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2 Comments

Jean Over a year ago

Why does p.communicate block though, when you pass a string? It could close pipe internally since we are passing a string..right?

Mad Physicist Over a year ago

@Jean. That's just what it's designed and documented to do. It optionally sends a single string, then waits for the process so close its stdout (usually by terminating). There's no particular overarching reason, except that that's a very common way to use processes. Perhaps it's the choice of name that's confusing?

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