Exercise 10: Two Channels and a Countdown — Possible Solution
====================================================================

import sys
import time

tasks = [("backup", True), ("upload", False), ("cleanup", True)]

failures = 0
for name, succeeded in tasks:
    if succeeded:
        print(f"OK: {name}", flush=True)
    else:
        print(f"FAILED: {name}", file=sys.stderr, flush=True)
        failures += 1

print(f"{len(tasks) - failures} of {len(tasks)} tasks succeeded", flush=True)

for n in (3, 2, 1):
    print(n, end=" ", flush=True)
    time.sleep(0.2)
print("Go!")

Output:
OK: backup
FAILED: upload
OK: cleanup
2 of 3 tasks succeeded
3 2 1 Go!

WHY THIS WORKS AS AN ANSWER
------------------------------
file=sys.stderr sends a line to the error channel instead of the
normal one. In a terminal both appear on screen, but a program can
redirect them separately, for example saving normal output to a file
while errors still show on the screen. flush=True writes each line
out immediately; without it normal output is held in a buffer and
can appear after later error lines when the two are redirected
together. The countdown uses end=' ' to stay on one line, and
flush=True is essential there: without it the numbers could sit in
the buffer and all appear together at the end rather than one every
fraction of a second. The final bare-argument print('Go!') supplies
the newline.