Why python don't print after a time.sleep()?

Question

I'm programming in python for almost 2 years and I found a really weird thing when watching some old code.

import random, sys, time

try:
    while True:
        print(' ', str(random.randint(100,999)), end='')
        time.sleep(0.01)
except:
    sys.exit()

Following the syntax of this code my program should print a space and a random number from 100 to 999 forever, but this isn't happening.

When I run this code nothing appear on the screen until I press CTRL-C, even removing the try statement din't change anything.

I tried changing console (Windows terminal, powershell and CMD) but nothing happened.

Can please someone help me out with this? Thanks

Answer 1

Python keeps your print outputs in a buffer and waits for the end of the line before actually displaying it. To force the display in Python 3, you can add the keyword flush=True.

import random, sys, time

try:
    while True:
        print(' ', str(random.randint(100,999)), end='', flush=True)
        time.sleep(0.01)
except:
    sys.exit()

Alternatively, you can:

• call sys.stdout.flush() after the print (Python 2-compatible);
• use the -u flag when executing the script (python -u script.py);
• set the environment variable PYTHONUNBUFFERED to TRUE.