Using multiple processes is simpler in terms of locks etc, but python libraries like multiprocessing or even subprocess.popen[1] which make using multiple processes seem easy are full of footguns which cause deadlocks due to fork-safe code not being well understood. I’ve seen this lead to code ‘working’ and being merged but then triggering sporadic deadlocks in production after a few weeks.

The default for multiprocessing is still to fork (fortunately changing in 3.14), which means all of your parent process’ threaded code (incl. third party libraries) has to be fork-safe. There’s no static analysis checks for this.

This kind of easy to use but incredibly hard to use safely library has made python for long running production services incredibly painful in my experience.

[1] Some arguments to subprocess.popen look handy but actually cause python interpreter code to be executed after the fork and before the execve, which has caused production logging-related deadlocks for me. The original author was very bright but didn’t notice the footgun.