Why do __setattr__ and __delattr__ raise an AttributeError in this case?

In Python, what is the rationale for which object.__setattr__ and type.__setattr__ raise an AttributeError during attribute update if the type has an attribute which is a data descriptor without a __set__ method? Likewise, what is the rationale for which object.__delattr__ and type.__delattr__ raise an AttributeError during attribute deletion if the type has an attribute which is a data descriptor without a __delete__ method?

I am asking this because I have noticed that object.__getattribute__ and type.__getattribute__ do not raise an AttributeError during attribute lookup if the type has an attribute which is a data descriptor without a __get__ method.

Here is a simple program illustrating the differences between attribute lookup by object.__getattribute__ on the one hand (AttributeError is not raised), and attribute update by object.__setattr__ and attribute deletion by object.__delattr__ on the other hand (AttributeError is raised):

class DataDescriptor1:  # missing __get__
    def __set__(self, instance, value): pass
    def __delete__(self, instance): pass

class DataDescriptor2:  # missing __set__
    def __get__(self, instance, owner=None): pass
    def __delete__(self, instance): pass

class DataDescriptor3:  # missing __delete__
    def __get__(self, instance, owner=None): pass
    def __set__(self, instance, value): pass

class A:
    x = DataDescriptor1()
    y = DataDescriptor2()
    z = DataDescriptor3()

a = A()
vars(a).update({'x': 'foo', 'y': 'bar', 'z': 'baz'})

a.x
# actual: returns 'foo'
# expected: returns 'foo'

a.y = 'qux'
# actual: raises AttributeError: __set__
# expected: vars(a)['y'] == 'qux'

del a.z
# actual: raises AttributeError: __delete__
# expected: 'z' not in vars(a)

Here is another simple program illustrating the differences between attribute lookup by type.__getattribute__ on the one hand (AttributeError is not raised), and attribute update by type.__setattr__ and attribute deletion by type.__delattr__ on the other hand (AttributeError is raised):

class DataDescriptor1:  # missing __get__
    def __set__(self, instance, value): pass
    def __delete__(self, instance): pass

class DataDescriptor2:  # missing __set__
    def __get__(self, instance, owner=None): pass
    def __delete__(self, instance): pass

class DataDescriptor3:  # missing __delete__
    def __get__(self, instance, owner=None): pass
    def __set__(self, instance, value): pass

class M(type):
    x = DataDescriptor1()
    y = DataDescriptor2()
    z = DataDescriptor3()

class A(metaclass=M):
    x = 'foo'
    y = 'bar'
    z = 'baz'

A.x
# actual: returns 'foo'
# expected: returns 'foo'

A.y = 'qux'
# actual: raises AttributeError: __set__
# expected: vars(A)['y'] == 'qux'

del A.z
# actual: raises AttributeError: __delete__
# expected: 'z' not in vars(A)

I would expect the instance dictionary to be mutated instead of getting an AttributeError for attribute update and attribute deletion. Attribute lookup returns a value from the instance dictionary, so I am wondering why attribute update and attribute deletion do not use the instance dictionary as well (like they would do if the type did not have an attribute which is a data descriptor).

I think it's just a consequence of the C-level design that no one really thought or cared much about.

At C level, __set__ and __delete__ correspond to the same C-level slot, tp_descr_set, and deletion is specified by passing a null value to set. (This is similar to the design used for __setattr__ and __delattr__, which also correspond to a single slot that also gets passed NULL for deletion.)

If you implement either __set__ or __delete__, the C-level slot gets set to a wrapper function that looks for __set__ or __delete__ and calls it:

static int
slot_tp_descr_set(PyObject *self, PyObject *target, PyObject *value)
{
    PyObject* stack[3];
    PyObject *res;
    _Py_IDENTIFIER(__delete__);
    _Py_IDENTIFIER(__set__);

    stack[0] = self;
    stack[1] = target;
    if (value == NULL) {
        res = vectorcall_method(&PyId___delete__, stack, 2);
    }
    else {
        stack[2] = value;
        res = vectorcall_method(&PyId___set__, stack, 3);
    }
    if (res == NULL)
        return -1;
    Py_DECREF(res);
    return 0;
}

The slot has no way to say "oops, didn't find the method, go back to normal handling", and it doesn't try. It also doesn't try to emulate the normal handling - that would be error-prone, since "normal handling" is type-dependent, and it can't know what to emulate for all types. If the slot wrapper doesn't find the method, it just raises an exception.

This effect wouldn't happen if __set__ and __delete__ had gotten two slots, but someone would have had to care while they were designing the API, and I doubt anyone did.