Hum, I had to patch asyncio to avoid a ResourceWarning because asyncio polls the status of the child process using a child watcher, whereas the Popen object is not aware of the child watcher:

New changeset 72946937536e by Victor Stinner in branch '3.5':
asyncio: fix ResourceWarning related to subprocesses
https://hg.python.org/cpython/rev/72946937536e

The asyncio child watcher uses os.waitpid() on a specific pid or wait for the exit of any child process depending on the chosen implemetation. The fast watcher implementations uses a signal handler on the signal SIGCHLD.