Daemon threads vs daemon processes in Python

Question

Based on thePython documentation , daemon threads are threads that die once the main thread dies. This seems to be the complete opposite behavior of daemon processes which involve creating a child process and terminating the parent process in order to have init take over the child process (aka killing the parent process does NOT kill the child process).

So why do daemon threads die when the parent dies, is this a misnomer? I would think that "daemon" threads would keep running after the main process has been terminated.

Answer 1

It's just names meaning different things in different contexts.

In case you are not aware, like threading.Thread , multiprocessing.Process also can be flagged as "daemon". Your description of "daemon processes" fits to Unix-daemons, not to Python's daemon-processes.

The docs also have a section about Process.daemon :

... Note that a daemonic process is not allowed to create child processes. Otherwise a daemonic process would leave its children orphaned if it gets terminated when its parent process exits. Additionally, these are not Unix daemons or services, they are normal processes that will be terminated (and not joined) if non-daemonic processes have exited.

The only thing in common between Python's daemon-processes and Unix-daemons (or Windows "Services") is that you would use them for background-tasks (for Python: only an option for tasks which don't need proper clean up on shutdown, though).

Python imposes it's own abstraction layer on top of OS-threads and processes. The daemon-attribute for Thread and Process is about this OS-independent , Python-level abstraction.

At the Python-level, a daemon-thread is a thread which doesn't get joined (awaited to exit voluntarily) when the main-thread exits and a daemon-process is a process which gets terminated (not joined) when the parent-process exits. Daemon-threads and processes both experience the same behavior in that their natural exit is not awaited in case the main or parent-process is shutting down. That's all.

Note that Windows doesn't even have the concept of "related processes" like Unix, but Python implements this relationship of "child" and "parent" in a cross-platform manner.

I would think that "daemon" threads would keep running after the main process has been terminated.

A thread cannot exist outside of a process. A process always hosts and gives context to at least one thread.