Using ANSI Escape Sequence in Python on sys.stderr seems to affect incorrect output line on screen

Question

I need to print a couple of error messasges to the screen and then erase them without disturbing the subsequent output to the user. I'm using ANSI Escape Sequences for cursor control. The code works if I print the error messages to sys.stdout but fails if sys.stderr is used.

Here is a sample code:

import sys, os
import time
os.system("")

errors = ["Error1", "Error2"]

for e in errors:
    print(e, file=sys.stderr)
    time.sleep(1)
    print("\033[F\033[2K\033[1G", file=sys.stderr, end='')
    # print("\033[F\033[2K\033[1G", end='')  # This gives desired result

print("Standard output")

I expected the screen to display each error message for a second and then erase them. At the end, the screen should just display "Standard output". Testing the code on Python 3.11, the screen actually shows "Error2" at the end.

Note: Changing the file=sys.stderr to file=sys.stdout yields desired result but I was hoping to display errors on a separate channel from the usual output.

[Relevant](https://stackoverflow.com/questions/19990589/stderr-and-stdout-not-buffered-vs-buffered)? — Dennis Williamson, Jun 23 '23 at 11:56
Also, you may want to use the [curses](https://docs.python.org/3/library/curses.html) module instead of hard coded sequences. — Dennis Williamson, Jun 23 '23 at 12:03
@DennisWilliamson Thanks. Yes, I believe it is related. Adding sys.stderr.flush() before the standard output did the trick. — Elvis M, Jun 23 '23 at 12:06
Add *flush=True* for consistent behaviour. One stream is buffered, the other is not — DarkKnight, Jun 23 '23 at 12:06

Using ANSI Escape Sequence in Python on sys.stderr seems to affect incorrect output line on screen

0 Answers0