Currently, support.threading_cleanup() waits 1 second before emitting a warning about threads running in the background. I propose to emit immediately the warning to be able to catch bugs quicker, bugs like bpo-31233 or bpo-30830. The problem is that currently, 12 test files still leak threads: --- 12 tests altered the execution environment: test_asyncio test_concurrent_futures test_fork1 test_httpservers test_logging test_pydoc test_ssl test_thread test_threaded_import test_threading test_wait3 test_xmlrpc --- Since threading_cleanup() warnings now mark tests as failed, we should first fix these tests before being able to make support.threading_cleanup() stricter.