How to grab python output in console?

Question

linux

Here is an example script

import logging
from time import sleep
   
while True: 
    print('Hello') 
    logging.warning('I told you so')  
    logging.warning('Watch out!')     
    sleep(1)

python main.py 2>&1 | grep Watch

And everything works.


But such number with print any more does not transit.
python main.py 2>&1 | grep Hello
We will get an empty console.

Is there a way to filter prints?

Answer 1

Python buffers output by default - it doesn't print to stdout right away.
If you wait a little longer, then grep will then immediately print a bunch of Hello when the buffer is full and will be output to stdout in its entirety.
You can do this at the time you want by calling sys.stdout.flush().