This makes it impossible to port certain types to the stable ABI and remove statics from the interpreter.

New changeset 3368f3c6ae4140a0883e19350e672fd09c9db616 by Dino Viehland (Eddie Elizondo) in branch 'master':
bpo-38140: Make dict and weakref offsets opaque for C heap types (#16076)
https://github.com/python/cpython/commit/3368f3c6ae4140a0883e19350e672fd09c9db616

Thanks for the feature!
This still needs documentation, however.

New changeset 468f8a6ee0d83e36795bbab6f20037b0e380f493 by Petr Viktorin in branch 'master':
bpo-38140: Document offsets in PyMemberDef (GH-16354)
https://github.com/python/cpython/commit/468f8a6ee0d83e36795bbab6f20037b0e380f493

Got an compile error, i have not check the detail for now:

python: Objects/typeobject.c:2867: PyType_FromSpecWithBases: Assertion `memb->type == 19' failed.

i report it in bpo 38273

This line in _tesctcapimodule.c:
    {"__dictoffset__", T_PYSSIZET, -sizeof(void*), READONLY},

gives a legitimate-sounding warning on a Windows buildbot:

         d:\buildarea\3.x.ware-win81-release.nondebug\build\modules\_testcapimodule.c(6409): warning C4146: unary minus operator applied to unsigned type, result still unsigned [D:\buildarea\3.x.ware-win81-release.nondebug\build\PCbuild\_testcapi.vcxproj]

Hey Petr, I'll get to document this and fix the windows warning over the weekend. I'll ping you on Github once it's ready

Woops, just realized that you already documented this, thanks! Btw, Victor already merged a fix for the windows compiler warning. This issue can be closed now

Thank you all!