The following code causes an assertion failure in PyErr_WriteUnraisable() (in
Python/errors.c):

class BadException(Exception):
    __module__ = None

class BadClass:
    def __del__(self):
        raise BadException

foo = BadClass()
del foo

this is because PyErr_WriteUnraisable() assumes that __module__ is a string,
and passes it to _PyUnicode_EqualToASCIIId(), which asserts it received a string.

what is the wanted behavior in such a case?
should we ignore the bad __module__ and print '<unknown>' as the module name
in the traceback?

Printing '<unknown>' LGTM.

Implicit converting to str can raise a warning or exception if __module__ is a bytes object.

I think this patch doesn't need a test (which would require using a subprocess).

what do you mean by 'Implicit converting to str can raise a warning or exception if __module__ is a bytes object.'?

should we treat __module__ differently in case it is a bytes object?

PyFile_WriteObject(moduleName, f, Py_PRINT_RAW) implicitly converts its argument to string. I mean that treating non-string moduleName the same way as string moduleName not equal to string "builtins" and calling PyFile_WriteObject() would cause other problem. Treating non-string moduleName the same way as moduleName==NULL LGTM.

New changeset f6e61df by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31418: Fix an assertion failure in PyErr_WriteUnraisable() in case of an exception with a bad __module__ attribute. (bpo-3539)
f6e61df

New changeset 5dbb28e by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31418: Fix an assertion failure in PyErr_WriteUnraisable() in case of an exception with a bad __module__ attribute. (GH-3539) (bpo-3556)
5dbb28e