Implement modules as singletons Python semantics.
In Python, importing module several times returns same underlying module object. This also fixes import statement handling for builtin modules. There're still issues: 1. CPython exposes set of loaded modules as sys.modules, we may want to do that either. 2. Builtin modules are implicitly imported, which is not really correct. We should separate registering a (builtin) module and importing a module. CPython keeps builtin module names in sys.builtin_module_names .