But one big problem with this approach is how to share Python objects between interpreters. Sharing raw data, like streams of bytes, isn’t difficult, but it’s also not very useful.

This is to ensure only one thread at a time can modify Python objects, so data doesn’t get corrupted. Python threads aren’t good for running CPU-bound tasks side by side (at least not for now).

So, you might want to use threads. And indeed, Python's threads work, and they work well, for many tasks. But they aren't as good as they might be, because of the GIL (the global interpreter lock), ...