It seems some code in _PyObject_GenericSetAttrWithDict is not necessary. There is no need to check data descriptor again using PyDescr_IsData. And the second if (f != NULL) {} will never function.

LGTM. Had to check the definition of PyDescr_IsData to determine that checking the value from tp_descr_set for NULL was exactly what that macro does, but yeah, it looks like the first test was redundant, and f is never assigned outside that block, so the second block after the rest of the work is pointless; you'd never get there.

This code mirrors code in the Get function, but that function inspects tp_descr_get (not set) instead. As I understand it, this is the difference between “data” descriptors and “non-data” descriptors. So I think the change is okay.

LGTM. But there are other redundant code in _PyObject_GenericSetAttrWithDict(). Here is a patch that makes larger refactoring.

It's a better work. And the code looks simpler now. I test it with the test suite and none fails (though some tests are skipped due to platform).

Serhiy’s version looks good to me

I'll defer committing the patch until some progress in issue26767 will be reached. May be _PyObject_GenericSetAttrWithDict() will have to rewrite again.

New changeset e5149789e4ea by Serhiy Storchaka in branch 'default':
Issue #26745: Removed redundant code in _PyObject_GenericSetAttrWithDict.
https://hg.python.org/cpython/rev/e5149789e4ea

Issue26767 looks too complex and resolving it needs much more rewriting.