Python not printing values in loop

Question

Here is a sample code that I tried running:

import sys
while True:
    print "Hello World"
    #sys.stdout.write("Hello World")
    #sys.stderr.write("Hello World")
    sys.stdout.flush()

This runs fine and prints "Hello World" when ran from the python interpreter. The problem is when I try to run it as follows and writing to a pipe does not work with print and stdout even if I flush the output. It only works when I uncomment the sys.stderr.write().

python test.py | tail -n 1

Kindly note that it only occurs when the print statement is in a loop. Can anyone help me understand why it behaves this way?

Thanks

Why describe the issue rather than provide runnable code that illustrates the thing that surprises you? — roganjosh
– roganjosh, Commented Oct 22, 2017 at 22:36

Iguananaut · Accepted Answer · 2017-10-22 22:39:05Z

4

You're producing endless output and tail -n 1 is waiting for the last line of input from stdin, which never arrives.

answered Oct 22, 2017 at 22:39

Iguananaut

23.8k6 gold badges54 silver badges65 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Anirudh A Over a year ago

Thanks @Iguananaut for the help. It seems I was overthinking. But as a followup, anything but last line will be written to standard error? I have accepted your answer as my primary question has been answered.

Iguananaut Over a year ago

I'm not sure--that sounds tricky and might not be doable as a one-liner. Though if you ask that as a separate question I'm sure there's a bash wizard out there who can answer it.

Collectives™ on Stack Overflow

Python not printing values in loop

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related