Unable to pipe python output to program

Question

I want to pipe stdout from python to another program, but I am faceing a problem where my stdout is not piped.

I have it shortened down to a simple sample:

import time

while True:
    print("Hello!")
    time.sleep(1)

I check it with cat like so:

./my_python_script.py | cat

And then I get nothing at all.

Strange thing is that it works just fine if i remove the sleep command. However, I do not want to pipe the output that fast, so I would really like to sleep for a second.

I checked with the corresponding bash script:

while true; do
    echo "Hello!"
    sleep 1
done

And that works like a charm too. So any idea as to why the python script does not pipe the output as expected? Thanks!

By default, pipes only send data when their buffer fills up (usually 8K). Your example would have worked eventually, if you had waited long enough. Your non-sleep example worked because it filled up the buffer immediately. — John Gordon
– John Gordon, Commented Aug 29, 2016 at 18:16

Robert Seaman · Accepted Answer · 2016-08-29 18:13:44Z

15

You'll need to flush the stdout:

import time
import sys

while True:
    print("Hello!")
    sys.stdout.flush()
    time.sleep(1)

answered Aug 29, 2016 at 18:13

Robert Seaman

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1 Comment

smac89 Over a year ago

Nice answer. I had a feeling it had to do with buffer not filling up fast enough

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