It seems like an unit test of test_thread leaks a process. A bisection found the following 2 tests are enough to get the warning: * test.test_thread.TestForkInThread.test_forkinthread * test.test_thread.LockTests.test_acquire_contended I bet that test_forkinthread() doesn't wait correctly for the child process (fork) completion using os.waitpid() or something like that. Example 1 (from Refleak buildbot): haypo@selma$ ./python -m test -R 3:3 test_thread Run tests sequentially 0:00:00 load avg: 0.55 [1/1] test_thread beginning 6 repetitions 123456 Warning -- reap_children() reaped child process 6984 .Warning -- reap_children() reaped child process 7054 .Warning -- reap_children() reaped child process 7125 .Warning -- reap_children() reaped child process 7195 .Warning -- reap_children() reaped child process 7266 .Warning -- reap_children() reaped child process 7336 . 1 test OK. Total duration: 5 sec Tests result: SUCCESS Example 2 (only run the 2 methods): haypo@selma$ ./python -m test -m test.test_thread.TestForkInThread.test_forkinthread -m test.test_thread.LockTests.test_acquire_contended -v test_thread test_thread 0:00:00 load avg: 0.27 [1/2] test_thread test_acquire_contended (test.test_thread.LockTests) ... ok test_forkinthread (test.test_thread.TestForkInThread) ... ok ---------------------------------------------------------------------- Ran 2 tests in 0.037s OK 0:00:00 load avg: 0.27 [2/2] test_thread test_acquire_contended (test.test_thread.LockTests) ... Warning -- reap_children() reaped child process 10613 ok test_forkinthread (test.test_thread.TestForkInThread) ... ok ---------------------------------------------------------------------- Ran 2 tests in 0.036s OK test_thread failed (env changed)
This issue has been migrated to GitHub: https://github.com/python/cpython/issues/75333 