How to write Python scripts that work with Linux pipes?

Question

In speaker.py, I use print to output text to STDOUT:

import time

while True:
    time.sleep(1)
    print("hello")

And in listener.py, I use input to read from STDIN:

while True:
    line = input()
    if not line:
        break
    print(line)

I'm trying to connect these two scripts with a pipe:

python speaker.py | python listener.py

But listner.py output nothing.

What's wrong?

Answer 1

Nothing is wrong per se, but you bumped into buffering. Take out the sleep and you should see output pretty much immediately.

http://mywiki.wooledge.org/BashFAQ/009 is nominally a Bash question, but applies to any Unix-type I/O, and explains the issues thoroughly.

Answer 2

An alternative to re-opening stdout as Andrea mentioned is to start up Python in unbuffered mode with the -u option:

python -u speaker.py | python -u listener.py