How to practise with CCC past papers

# Our local checker: runs a program on every .in file in a folder and
# compares its output with the matching .out file, exactly, like a grader.
# Usage: python judge.py tests python cold_days.py
import subprocess
import sys
import time
from pathlib import Path

LIMIT = 3.0  # seconds per test, the CCC default

def verdict(cmd, test_in, test_out):
    start = time.time()
    try:
        run = subprocess.run(cmd, stdin=open(test_in), capture_output=True,
                             text=True, timeout=LIMIT)
    except subprocess.TimeoutExpired:
        return "time-limit exceeded"
    if run.returncode != 0:
        return "run-time error"
    got = run.stdout.rstrip("\n").split("\n")
    want = test_out.read_text().rstrip("\n").split("\n")
    if got != want:  # strict on purpose: even a trailing space fails
        return "wrong answer"
    return "correct (%.2f s)" % (time.time() - start)

folder, cmd = Path(sys.argv[1]), sys.argv[2:]
for test_in in sorted(folder.glob("*.in")):
    print(test_in.name, verdict(cmd, test_in, test_in.with_suffix(".out")))

Put each test in a folder as a pair of files, 01.in with the input and 01.out with the expected output. The checker gives each test three seconds, the CCC's default, and treats any difference, even a trailing space, as a wrong answer.

tests/01.in
5
-3 2 0 -11 4

tests/01.out
2

# Our practice problem: Cold Days (not a CEMC problem)
n = int(input())
temps = list(map(int, input().split()))
print(sum(1 for t in temps if t < 0))

# The same program with a friendly prompt: the grader marks it wrong
n = int(input("How many days? "))
temps = list(map(int, input().split()))
print(sum(1 for t in temps if t < 0))

$ python judge.py tests python cold_days.py
01.in correct (0.03 s)
02.in correct (0.03 s)
03.in correct (0.03 s)

$ python judge.py tests python cold_days_prompt.py
01.in wrong answer
02.in wrong answer
03.in wrong answer

Both programs count correctly. The second fails every test because the words "How many days? " are printed to standard output in front of the answer. The CCC rules forbid exactly this: output must match the problem's format, with no prompts. Times will differ on your computer.