The following code causes an assertion failure: import os os.__name__ = None os.does_not_exist this is because module_getattro() (in Objects/moduleobject.c) assumes that __name__ is a string, and passes it to PyErr_Format(), which asserts it is a string. if we fixed that one (so that the code above would raise an AttributeError), the following code would still cause an assertion failure: import os os.__name__ = None from os import does_not_exist this is because import_from() (in Python/ceval.c) also assumes that __name__ is a string, and passes it to PyUnicode_FromFormat(), which asserts it is a string. BTW, while we are in module_getattro(): isn't the second call to PyErr_Clear() redundant? (Ethan, IIUC, you worked on this as part of #8297 some years ago..)

 New changeset 6db7033192cd537ca987a65971acb01206c3ba82 by Serhiy Storchaka (Oren Milman) in branch 'master': bpo-31492: Fix assertion failures in case of a module with a bad __name__ attribute. (#3620) https://github.com/python/cpython/commit/6db7033192cd537ca987a65971acb01206c3ba82 

 New changeset f0db2dfda777ad3380e7816cabe4c4240f31687f by Serhiy Storchaka in branch '3.6': [3.6] bpo-31492: Fix assertion failures in case of a module with a bad __name__ attribute. (GH-3620). (#3773) https://github.com/python/cpython/commit/f0db2dfda777ad3380e7816cabe4c4240f31687f