> The process is not polled to be able to emit the warning in more 
> cases, again, to ease debug.

It emits an incorrect warning if the process has already exited: "subprocess %s is still running". This can be rectified. Here's my current understanding:

    def __del__(self, _maxsize=sys.maxsize, _warn=warnings.warn):
        if not self._child_created or self.returncode is not None:
            return
        # In Unix, not reading the subprocess exit status creates a zombie
        # process, which is only destroyed at the parent Python process exit.
        # In Windows, no one else should have a reference to our _handle, so
        # it should get finalized and thus closed, but we use the same warning
        # in order to consistently educate developers.
        if self._internal_poll(_deadstate=_maxsize) is not None:
            _warn("subprocess %s was implicitly finalized" % self.pid,
                  ResourceWarning, source=self)
        else:
            _warn("subprocess %s is still running" % self.pid,
                  ResourceWarning, source=self)
            # Keep this instance alive until we can wait on it, if needed.
            if _active is not None:
                _active.append(self)