EC
ProcessLookupError
PythonERRORNotableOS ErrorHIGH confidence

Process not found

Production Risk

Common in process management code; always handle the case that a PID may no longer exist.

What this means

A subclass of OSError (errno ESRCH) raised when a process cannot be found — typically when sending a signal to a PID that no longer exists.

Why it happens
  1. 1os.kill() called with a PID that has already exited
  2. 2Race condition: process exited between PID lookup and signal send
How to reproduce

Sending a signal to a process that has already exited.

trigger — this will error
trigger — this will error
import os
os.kill(99999, 0)  # PID 99999 does not exist

expected output

ProcessLookupError: [Errno 3] No such process

Fix

Catch ProcessLookupError when signaling processes

WHEN Sending signals to externally managed processes

Catch ProcessLookupError when signaling processes
import os

def is_running(pid):
    try:
        os.kill(pid, 0)  # Signal 0 checks existence without killing
        return True
    except ProcessLookupError:
        return False
    except PermissionError:
        return True  # Process exists but we lack permission

Why this works

os.kill(pid, 0) is the standard way to check if a process exists; signal 0 does not kill.

Code examples
Triggerpython
import os
os.kill(99999, 0)  # ProcessLookupError: No such process
Handle with try/exceptpython
import os
try:
    os.kill(pid, 0)
except ProcessLookupError:
    print(f"Process {pid} no longer exists")
Avoid by using signal 0 for existence checkpython
def is_running(pid):
    try:
        os.kill(pid, 0)
        return True
    except (ProcessLookupError, PermissionError):
        return False
Same error in other languages
ESRCHin Linux / POSIX→
Sources
Official documentation ↗

Python Docs — Built-in Exceptions

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