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

score 6 · Accepted Answer · answered Nov 15 '19 at 19:40

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.

score 5 · Answer 2 · answered Nov 15 '19 at 19:34

5

This answer Python sleep() inhibit print with comma? suggests adding sys.stdout.flush() between the print and the sleep. Tested it - works.

answered Nov 15 '19 at 19:34

Iliyan Bobev

3,070
2
20
24

Thanks for your answer, i gave the tick to another user to help him grow on this website – user187624 Nov 16 '19 at 14:58

score 0 · Answer 3 · answered Nov 15 '19 at 19:36

0

Try this:

import random, sys, time
try:
    while True:
        print(' ', str(random.randint(100,999)), end='', flush=True)
        time.sleep(1)
except:
    sys.exit()

answered Nov 15 '19 at 19:36

Akshay G Bhardwaj

339
1
4
14

score 0 · Answer 4 · answered Nov 15 '19 at 21:39

0

Which version of PYTHON are you using? I was able to run your code in my PC where I am using PYTHON 3.7 in PYCHARM 2019.2.4.
It also ran successfully on PYTHON 3.7.5 shell

answered Nov 15 '19 at 21:39

Shahriar Rahman Zahin

624
1
11
23

I'm using Python 3.7 on Windows 10 and on Android 10 (beta) – user187624 Nov 16 '19 at 14:54

score -1 · Answer 5 · answered Nov 15 '19 at 19:28

-1

This is because you have a while True clause that's looping forever, Causing it loop recursively infinitely.

Here is the updated code

import random, sys, time

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

I added a break statement to move to get out of the loop.

answered Nov 15 '19 at 19:28

Salman Farsi

466
3
13

2

I don't think it's solving the problem OP has. Your code only prints the number once. – Arn Nov 15 '19 at 19:35
1

There is no recursion. – quamrana Nov 15 '19 at 19:37
Oh, I'm afraid I had misunderstood the question. – Salman Farsi Nov 15 '19 at 19:46
I think you should remove this answer – user187624 Nov 16 '19 at 14:53

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

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