The sys module is a rather special case as far as modules go. It is effectively a "console" into the interpreter's internal state and that includes some mutable state. Since it is a module, we don't have much of an opportunity to: * validate values assigned to its attributes [1] * issue DeprecationWarning for deprecated attrs [2] * alias attrs [2] * replace get (and get/set) functions with properties * re-organize sys [3] One possible solution I've been toying with for quite a while [2] is to rename the current sys module "_sys" and then add Lib/sys.py. The new module would proxy the old one (for backward-compatibility), but also allow us to do all of the above (e.g. validate sys.modules). I implemented this a few weeks ago: https://github.com/ericsnowcurrently/cpython/tree/sys-module (for the sake of comparison: https://github.com/ericsnowcurrently/cpython/pull/2) It uses the trick of replacing itself in sys.modules (though it could just as well set __class__). The only problem I've encountered is code that uses "type(sys)" to get types.ModuleType. Under my branch "type(sys)" returns the ModuleType subclass. Otherwise everything looks fine. Thoughts? -eric [1] I ran into this recently: https://bugs.python.org/issue31404 [2] I have plans for a low-level encapsulation of the import state and a high-level API for the current import machinery as a whole. As part of that I'd like to be able to deprecate the current import state attrs (e.g. sys.modules) and make them look up values from sys.importstate. [3] This has come up before, including https://www.python.org/dev/peps/pep-3139/. Also PEP 432 implies a clearer structure for sys.