Say you do "import smtplib" in your main file. Now that module is imported -- forever. I don't know the internals of Python well enough, but I bet that the module reference has strong references to its contents, so that even if nobody is actually calling anything in smtplib, it will be there in case someone does. The same should be true about modules importing other modules; they stay visible at the scope-level, so they are permanently loaded.

So for those cases it would make sense to keep them separate from global garbage collection. I'm pretty sure that the method tables of all the classes in the system take up considerable space, probably in the order of megabytes for many apps.