Python carriage return and thread

Question

So I have a script that will continuously read from a sensor, print a carriage return and then the sensor value. It does this until raw_input() finishes blocking (enter pressed).

However, when I run it, instead of an increasing number, I see blank space. When I press enter, one number is printed and then the program exits. If \r is replaced by \n, the program runs as it should, printing out the "sensor" value (i = i + 1 is a placeholder for the reading of the sensor), but instead of reusing the same line it prints it on a new line. Why does it not work with \r? Here is the full code:

from threading import Thread
from time import sleep
import sys

running = True

def loop():
        i = 0
        while running:
                sys.stdout.write("\r" + str(i))
                i = i+1
                sleep(0.1)


thread = Thread(target=loop)
thread.start()
raw_input()
running = False
thread.join()

Maybe you want to read this: http://stackoverflow.com/questions/1761051/difference-between-n-and-r — jean-loup, Jul 23 '14 at 13:07
using `\r` it prints on consecutive lines and using `\n` it prints a newline as it should. What output do you expect to see? — Padraic Cunningham, Jul 23 '14 at 13:10
@PadraicCunningham using `\r` on mine shows me nothing until i press enter, then I see one line and our closes. I should see at the beginning a single line that rewrites and increases, and stops when i press enter. — Luke Moll, Jul 23 '14 at 13:14
using `\r` I see `1True` etc.. I don't think you need the global keyword either. — Padraic Cunningham, Jul 23 '14 at 13:17
@PadraicCunningham sorry, i left those in from when i was testing the code. What does it do now I've edited it? — Luke Moll, Jul 23 '14 at 13:21
same, `0...` then a newline for `\n` the output is on a single line for `\r` — Padraic Cunningham, Jul 23 '14 at 13:23
I edited my comment, the output is the same minus the True, the difference is the `\r` output is on a single line — Padraic Cunningham, Jul 23 '14 at 13:27
@PadraicCunningham the `\n` output i know works, does the `\r` output overwrite the same line or append it? — Luke Moll, Jul 23 '14 at 13:29

score 2 · Answer 1 · answered Jul 23 '14 at 13:30

The difference is that writing out the newline flushes the channel, while writing out a carriage return (or most other character) doesn't. You can get the same behaviour by adding an explicit flush after the write():

sys.stdout.flush()

As to why the lack of a flush causes the raw_input() to never return, I believe this is due to the buffering present on stdout; however, I've not been able to find more details.

score 1 · Answer 2 · answered Jul 23 '14 at 13:38

1

I was trying things and tried flushing the output with sys.stdout.flush() and it worked. Thank you all for your help.

answered Jul 23 '14 at 13:38

Luke Moll

428
6
18

Python carriage return and thread

2 Answers2