It's not possible (by intention) to instantiate a new instance of sys.flags. This is achieved by setting the "tp_new" slot to NULL (in `_PySys_Init()`), after `PyType_Ready()` is called, which means that a slot wrapper is added to the type dict for the "tp_new" slot (because the slot != NULL at that time). The problem is now that if one calls `sys.flags.__new__` directly, a null pointer dereference occurs in `tp_new_wrapper()`.

Attached is a patch that fixes the crash and adds a test.

I’m not sure it is useful to fix this bug.

Why not?

Because the class of sys.flags is an implementation detail.  Most people won’t try to instantiate it, IMO.

That's probably true, but IMHO it's not a valid reason to keep the buggy behavior.

Why would we want to prevent users from creating new instances of FlagsType?

You are right.  Even if it’s an undocumented internal type, there is no reason not to fix it.  There are plenty of similar crash fixes committed in the repo.

Thanks for the patch. You should add the same tests for sys.version_info and sys.getwindowsversion.

Thanks for reporting this and providing a patch, Trundle.

The Python 3 patch didn't apply cleanly anymore, so I regenerated it and added tests for sys.version_info and sys.getwindowsversion.

* The patch passes `make patchcheck`
* The full test suite passes with this patch.
* Testing manually, without this patch each of the following segfaults, and with the patch they raise errors instead:

import sys; sys.flags.__new__(type(sys.flags))
import sys; sys.version_info.__new__(type(sys.version_info))
import sys; sys.getwindowsversion().__new__(type(sys.getwindowsversion()))

One important caveat is that while I confirmed the sys.getwindowsversion segfault on Windows, I don't have Visual Studio set up so I couldn't build and test the new test for sys.getwindowsversion (I ran the full test suite on OSX, where this test is skipped).

Thanks for the patch, Jessica. It seems to work under Windows here.

New changeset 7052fdd90a11 by Antoine Pitrou in branch '3.4':
Issue #13204: Calling sys.flags.__new__ would crash the interpreter, now it raises a TypeError.
http://hg.python.org/cpython/rev/7052fdd90a11

New changeset a14012352f65 by Antoine Pitrou in branch 'default':
Issue #13204: Calling sys.flags.__new__ would crash the interpreter, now it raises a TypeError.
http://hg.python.org/cpython/rev/a14012352f65

I've committed the patch to 3.4 and 3.5. I'm closing the issue, I don't think fixing 2.7 is important at this point.