Here is a concrete example of ResourceWarning when debugging/stressing multiprocessing code, I interrupt (CTRL+c) test_resource_tracker while it's running:

vstinner@apu$ ./python -m test test_multiprocessing_spawn --fail-env-changed -v -m test_resource_tracker
== CPython 3.9.0a0 (heads/master:689830ee62, Jun 26 2019, 23:07:31) [GCC 9.1.1 20190503 (Red Hat 9.1.1-1)]
== Linux-5.1.11-300.fc30.x86_64-x86_64-with-glibc2.29 little-endian
== cwd: /home/vstinner/prog/python/master/build/test_python_18573
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.32 [1/1] test_multiprocessing_spawn
test_resource_tracker (test.test_multiprocessing_spawn.TestResourceTracker) ... 

^C

/home/vstinner/prog/python/master/Lib/subprocess.py:917: ResourceWarning: subprocess 18576 is still running
  _warn("subprocess %s is still running" % self.pid,
ResourceWarning: Enable tracemalloc to get the object allocation traceback
/home/vstinner/prog/python/master/Lib/test/libregrtest/runtest.py:283: ResourceWarning: unclosed file <_io.BufferedReader name=6>
  return INTERRUPTED
ResourceWarning: Enable tracemalloc to get the object allocation traceback

== Tests result: INTERRUPTED ==
Test suite interrupted by signal SIGINT.

1 test omitted:
    test_multiprocessing_spawn

Total duration: 304 ms
Tests result: INTERRUPTED