watchpid

watchpid is a tiny stdlib-only package for waiting until a process exits.

It uses OS process notifications when Python exposes them:

• Linux: os.pidfd_open()
• macOS/BSD: select.kqueue()
• Other platforms: polling fallback

Usage

Block until a pid exits:

from watchpid import wait_pid

if wait_pid(1234, timeout=5):
    print("exited")

Run a callback in a daemon thread:

from watchpid import watch_pid

watch_pid(1234, lambda: print("exited"))

Watch the parent pid set by jupyter_client:

from watchpid import watch_parent

watch_parent(lambda: print("parent exited"))

watch_parent() reads JPY_PARENT_PID by default. It returns None when the env var is unset, invalid, or <= 1 (so pid 1/init is never watched).

Caveats

• The event-based backends (pidfd, kqueue) detect exit reliably. The polling fallback uses pid liveness checks, so it is best-effort: it can't distinguish a reused pid, and an unreaped child process stays "alive" to the poller until it's wait()ed.

API

wait_pid(pid, timeout=None, poll=0.1) -> bool
watch_pid(pid, callback, timeout=None, poll=0.1, daemon=True, name=None) -> Thread
parent_pid(env="JPY_PARENT_PID") -> int | None
watch_parent(callback, env="JPY_PARENT_PID", timeout=None, poll=0.1, daemon=True, name="watchpid-parent") -> Thread | None

wait_pid() returns True when the process exits and False on timeout.

Development

pip install -e ".[test]"
pytest -q