Why does this Python code not print anything

Question

I was wondering, why does this Python code not print anything in my console?

from time import sleep
while True:
    print('#', end='')
    sleep(1)

If I remove the sleep function it works, and if I remove the end='' part it works too. I am using Python 3.9 and I have tested this with Dash, Bash and ZSH. I can achieve the desired output with the following code.

from time import sleep
hash = '#'
while True:
    print('\r' + hash, end='')
    hash = hash + '#'
    sleep(1)

Thank you in advance.

I just checked your code and it works well can you specify your problem again ? — Ata Reenes, Jan 08 '21 at 08:42
@AtaReenes nothing is showing on standard output until I interrupt the script from running. I already received a working answer, though. — Ray Bern, Jan 08 '21 at 08:45
its interesting which compiler or IDE are you using currently ? — Ata Reenes, Jan 08 '21 at 08:47
@AtaReenes I'm just running a script from the terminal, using the Python 3.9 interpreter. — Ray Bern, Jan 08 '21 at 08:48
as kwkt mentioned it's due to buffering many compilers solve that buzz by themselves that's why we normally dont cross with this bug . But yes for IDE flush=True can solve your problem. — Ata Reenes, Jan 08 '21 at 08:52
If you remove the `sleep`, it works the same way as before, but you see the output because the output buffers are flushed on program exit. — VPfB, Jan 08 '21 at 09:00

score 3 · Accepted Answer · answered Jan 08 '21 at 08:42

3

I would assume it's due to buffering. Try adding flush=True as one of the optional parameter to print.

answered Jan 08 '21 at 08:42

kwkt

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This worked, thank you. I will mark this as the answer as soon as it lets me do so. – Ray Bern Jan 08 '21 at 08:44
IMHO It would be nice if `flush=True` was automatically enabled when `end=...` is specified. (I understand that there is zero chance for such a change). – VPfB Jan 08 '21 at 08:52
@VPfB There's multiple ways you can go for that. 1, you can run Python with flag `-u` and it'll go unbuffered. 2, you can change the file descriptor `sys.stdout` to not buffer. Check out https://stackoverflow.com/q/107705/2327379 for more details. – kwkt Jan 08 '21 at 15:01

Ted Klein Bergman · Answer 2 · 2021-01-08T08:55:55.550

1

When using print, a newline character is added to your string unless you're overriding it (as you do) with the end parameter. For performance reasons, there exists an I/O buffer that only prints when it encounters a newline or the buffer fills up. Since your string doesn't contain a newline character anymore, you have to manually flush the buffer (i.e. send it to be printed).

from time import sleep
hash = '#'
while True:
    print('\r' + hash, end='', flush=True)
    hash = hash + '#'
    sleep(1)

edited Jan 08 '21 at 08:55

answered Jan 08 '21 at 08:52

Ted Klein Bergman

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@RayBern Sorry, I was wrong in my earlier comment. It seems to be that the buffer flush at the `\r` as well. – Ted Klein Bergman Jan 08 '21 at 09:02

Why does this Python code not print anything

2 Answers2