Proposed patch adds asserts for checking the type in macros PyTuple_GET_SIZE, PyList_GET_SIZE and PySet_GET_SIZE. This can help to find the misuse of these macros. Asserts already are used in macros PyBytes_GET_SIZE, PyByteArray_GET_SIZE, PyUnicode_GET_SIZE and PyDict_GET_SIZE. See also the discussion on Python-Dev: https://mail.python.org/pipermail/python-dev/2017-March/147628.html .

This change can break the code that uses these macros for setting the size. For example one place in odictobject.c. But I expect that such cases are rare. And all these macros are not in the limited API.

We should test to run popular C extensions like numpy, PyQt, cython, pillow, etc. with a patched Python to see if it's common to use a "GET" macro to "set" a size. Maybe test also Django see it probably uses many C extensions. I don't ask to test all these modules, but at least a few of them. By "testing", I mean run the test suite of these projects.

It would be enough to compile them. New macros produce compiler error when used as lvalue.

But I afraid it will take too much time on my netbook.

New changeset 1a5856bf9295fa73995898d576e0bedf016aee1f by Victor Stinner (Serhiy Storchaka) in branch 'master':
bpo-29867: Add asserts in PyTuple_GET_SIZE, PyList_GET_SIZE and PySet_GET_SIZE. (#751)
https://github.com/python/cpython/commit/1a5856bf9295fa73995898d576e0bedf016aee1f

I merged your PR. We are at the begining of the 3.7 cycle. Applications can be fixed before 3.7.

In the worst case, we can revert the change.

I like this enhancement, it should help to catch bugs ;-)