python `print` does not work in loop

Question

I have multi loops in together and a sleep in the most inner loop. for example:

from time import sleep

for i in range(10):
    print i,
    for j in range(-5,5):
        if j > 0:
            print '.',
        else:
            print 'D',
        sleep(1)
    print ''

if you run the code, you may expected to get i value after it D sleep 1 second and another D and again sleep until to the end.

but the result is difference, it waits 10 seconds and prints the whole line of 0 D D D D D D . . . . and waiting again to printing next line.

I found the comma at the end of printing causes this problem. How can I solve it?

what output do you expect, it prints a new line on each iteration for me — Padraic Cunningham, Aug 18 '14 at 17:29
every `j` iterate? it prints on every `i` iteration but I need prints on every `j` — Farhadix, Aug 18 '14 at 17:31

score 16 · Accepted Answer · answered Aug 18 '14 at 17:25

16

Because of existence of comma, the output buffers until a \n.

You should flush the stdout after every print or use sys.stdout.write and flush buffer.

Define your print method:

import sys

def my_print(text):
    sys.stdout.write(str(text))
    sys.stdout.flush()

and at the end of line print a \n

answered Aug 18 '14 at 17:25

Farhadix

1,395
2
12
25

2

it did not work with me, I tried flushing the output but nothing happens – Amado Saladino May 04 '20 at 12:42

Nir Alfasi · Answer 2 · 2023-08-02T21:05:43.343

3

The problem using print <something>, is buffering and printing only when the result is ready to be printed.

You can solve it using print_function from __future__ (which will be in compliance with Python 3 as well):

from __future__ import print_function
from time import sleep
import sys

for i in range(10):
    print(i, end='')
    for j in range(-5,5):
        if j > 0:
            print('.', end='')
        else:
            print('D', end='')
        sys.stdout.flush()
        sleep(1)            
    print('')

edited Aug 02 '23 at 21:05

answered Aug 18 '14 at 17:32

Nir Alfasi

53,191
11
86
129

12

You still need a `sys.stdout.flush()` to make it look like it's printing incrementally. If using Python 3.3+, then supplying a `flush=True` to print is also available. – Jon Clements Aug 18 '14 at 17:41
@JonClements thanks I wasn't aware of it since it works fine without the flush in python 2.7.5 – Nir Alfasi Aug 18 '14 at 17:46
1

It's `sys.stdout` that does the buffering. Using `print()` as a function does not change the buffering behaviour, but you *can* use `flush=True` in newer Python versions to force a flush. Use `sys.stdout.flush()` on versions where the `flush` argument is not available (like Python 2). – Martijn Pieters Sep 08 '17 at 11:58

Vishal Arya · Answer 3 · 2019-07-31T16:00:17.213

0

I would suggest override print just after import with below coe

import sys
def print_msg(*args,end='\n'):
    for item in args:
        sys.stdout.write(str(item)+' ')
    sys.stdout.write(end)
    sys.stdout.flush()

print = print_msg

edited Jul 31 '19 at 16:00

answered Jul 31 '19 at 10:04

Vishal Arya

21
5

python `print` does not work in loop

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