为什么 python 在 time.sleep() 之后不打印？

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

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

Answer 2

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 .

Answer 3

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

Answer 4

Try this:

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

Answer 5

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.