External library with python binding can't see env vars that were created during runtime

Question

I use an external library (OIIO with OCIO) which relies on env vars to resolve paths in its config.

When I set the env vars with os.environ and then use the library, it cannot resolve the paths; like it doesn't see the env vars.

I can make it work when:

• I set the env vars in the command line and calling the python script with python foo_bar.py
• or I set the env vars with os.environ and use subprocess.Popen() to call another python which uses the library
• or it also works with PyCharm when I set the env vars in the run configuration settings. PyCharm sets the env vars with os.environ then calls my .py with runfile('path_to.py', args=[...], wdir='path_to_working_dir') ('Cause I use "Run with Python Console")

So it seems it only works when I start a new process after setting the env vars. What is that extra step that starting these new processes does and using os.environ doesn't do? Can I fake / force that extra step?

Is there a way I can avoid using a new process? (I'd like to do so based on the 2nd answer in this thread).