Thread locals in Python - negatives, with regards to scalability?

Question

I'm wondering if there are some serious implications I might be creating for myself by using thread locals. I noticed that in the case of Flask, they use thread locals, and mention that it can cause issues with servers that aren't built with threads in mind. Is this an outdated concern? I'm using thread locals with Django for a few things, deploying with NGINX in front of UWSGI, or Gunicorn, on Ubuntu 10.04 with Postgres (not that the OS or DB probably matter, but just for clarity). Do I need to be worried?

Answer 1

Threadlocals aren't the most robust or secure way to do things - check out this note, for instance. [ Though also see Glenn's comment, below ]

I suppose if you have coded cleanly, with the idea that you're putting stuff into a big global pot of info, accepting unguaranteed data consistency in those threaded locals and taking care to avoid race conditions, etc, etc, you might well be ok.

But, even with that in mind, there's still the 'magic'ness of threaded local vars, so documenting clearly what the heck is going on and any time a threadedlocal var is used might help you/future developers of the codebase down the line.