Question

I would like to create a Pyramid app with an orm which I am writing (currently in deep alpha status). I want to plug the orm into the app sanely and thus I want to know how global objects are handled in multithreading.

In the file:
https://www.megiforge.pl/p/elephantoplasty/source/tree/0.0.1/src/eplasty/ctx.py
you can see, there is a global object called ctx which contains a default session. What if I run set_context() and start_session() in middleware at ingress? Can I expect then to have a separate session in ctx in every thread? Or is there a risk that two threads will use the same session?

Answer 1

Global variables are shared between all threads, so if you run those functions the threads will conflict with each other in unpredictable ways.

To do what you want you can use thread local data, using threading.local. You need to remove the global definition of ctx and then create the following function.

def get_ctx():
    thread_data = threading.local()
    if not hasattr(thread_data, "ctx"):
         thread_data.ctx = Ctx()
    return thread_data.ctx

Then, everywhere you reference ctx call get_ctx() instead. This will ensure that your context is not shared between threads.