The [language documentation](https://docs.python.org/3/reference/datamodel.html#invoking-descriptors) states:

> Data descriptors with __set__() and __get__() defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.

This override is not limited to data descriptors defining `__set__()` (and `__get__()`). It is also true for data descriptors defining `__delete__()` (and `__get__()`) and data descriptors defining both `__set__()` and `__delete__()` (and `__get__()`).

In other words, _any_ data descriptors (objects defining either `__set__()` or `__delete__()` or both) defining `__get__()` override an attribute redefinition in an instance dictionary.

I have opened a PR here: https://github.com/python/cpython/pull/16520

New changeset 4c155f738dc2e70ccb574f5169ad89c01753c6f7 by Raymond Hettinger (Géry Ogam) in branch 'master':
bpo-38336: Remove the __set__ method restriction on data descriptors for attribute lookup precedence (GH-16520)
https://github.com/python/cpython/commit/4c155f738dc2e70ccb574f5169ad89c01753c6f7